While Oliveria did not have a ready answer for this question, a ready answer to it does exist. It is even available as an infographic:
I have not been able to find the original source for this infographic. It has been floating around the web for several years. It is based on data collected by the Foreign Service Institute, which is where members of America's diplomatic corp learn new languages. It is obviously designed with the native English speaker in mind; the languages which are easiest to learn are those whose grammar, vocabulary, and writing systems are closest to English. What this infographic labels 'Language Proficiency' the State Department calls 'Minimum Professional Proficiency." The minimally proficient individual can listen to a song and understand its meaning, watch a film without subtitles, converse freely on most topics without preparation, and read a newspaper aimed at a general readership and understand most of what it means. They can talk well enough to make their meaning clear in most situations, though they cannot do this elegantly, and speak with a strong accent. They could probably take a college course in the language in question, but it would be mentally taxing in the extreme. You would not want this person interpreting for you in a legal setting. Writing an essay in the target language would require triple or quadruple the amount of time a native speaker would take to write the same amount. The end product would be readable, but obviously the product of a non-native writer.
The infographic has two measures of time: weeks and class hours. The second is the more important of the two. Students at the Foreign Service Institute are paid to learn the language they have been assigned; they spend close to five hours every day (except weekends) in class using the language. On the face of it it seems that most people cannot commit that sort of time to learning a new language. But that is not quite true--with a few brilliant savants excepted, everyone who becomes fluent in a new language spends just as much time learning their language of choice as the folks in the State Department do. To become fluent in Spanish you must actively use Spanish for more than 600 hours. Diplomats just pack those hours into fewer days than most people are willing to do.
Think of it as an equation. If the person studying Spanish wants to know how long it will take them to become professionally proficient in the language, all they need to do is figure out how many hours a week they are using their Spanish, and then divide the 600 hours needed for fluency by that number. This will leave them with a fair estimate of the number of weeks they will take to reach 'fluency.' Thus:
If you use Spanish for 25 hours of every week, you will become minimally proficient in 24 weeks, or a bit less than 6 months.
If you are using Spanish for 10 hours every week, you will become minimally proficient in 60 weeks, or a little bit more than a year.
If you are using Spanish for 4 hours every week, you will become minimally proficient in 150 weeks, or about three years.
I should be clear here what I mean by "using" a language. Any situation where the language learner must actively work to communicate or understand the target language counts. Conversation counts. Reading a book counts. Actively listening to a speech or a film or the radio--as opposed to letting it passively play in the background--counts. I submit that even the rote memorization of vocabulary lists and grammar structures counts, with the caveat that this will not be sufficient if it is all the language learner does. Class can count, if classes are structured to force the learner to actively use the language the entire class period. A new language is an alien intrusion on the brain. Your brain does not want to deal with it. Any activity that forces your brain to do so will help you here.
This is the hidden secret to language learning. Someone who spends 600 hours using cruddy textbooks and old fashioned methods will learn to speak a language better than someone who spends 100 hours with the world's best tutors, textbooks, and software. There are no real short cuts here. Yes, advances in cognition and linguistics can and should inform your language learning journey, and yes, it is easier to learn a new language with proper study materials than without them. However, nothing can replace being forced to speak, read, write, and listen to the language for hours on end, and real progress cannot be had without that. The temptation to over optimize your language learning process--to spend hour after hour clicking through blogs and websites on language learning, or reading review after review of different textbooks on Amazon.com--must be fought. In most cases you will be better off spending those precious hours simply studying the language and figuring out what works best for you as you go along.
Add all this together and you'll realize that the most important trait of the ideal language learner is discipline. It takes discipline to devote all of these hours to learning a new language, especially in its early stages, when speaking is embarrassing, reading seems pointless, listening destroys confidence, and memorizing new vocabulary is unbearably tedious. Occasionally you will meet people are so passionate about a language or a culture, or perhaps just so in love with language learning as whole, that they rush to their drills and their study lists like sailors on leave rush to shore. These people are found few and far between. For most people grit is what carries them through--or cheap replacements for grit, like fear of a failing a final.
"Immersion" is another common replacement for grit. Immersion is by no means a requirement for learning a second language. But for most people immersion is the best way to learn a language, and by now you should see why: in an immersive environment you do not have to rely on discipline and self control to force your brain to use the new language, because circumstances do that for you. If you do not learn how to buy food in the target language, you will go hungry. If you do not learn how to withdraw money from the bank, you will be penniless. If you do not learn how to find a bathroom, you will... well, you get the idea. A fully immersive environment can force you to use your new language every hour of the day. Most people go through their 600 hours quite quickly in such circumstances.
But not all people do. Occasionally you will meet expats who wonder why their language skills are barely any better now than when they arrived in the country, and the answer is almost certainly that they have figured out how to survive without speaking the target language. They usually spend all their time reading English language websites or hanging out with English speaking friends. The number of hours they spend using the language is hardly any different than the number of hours they were spending back at home. This is the story behind those old migrants you meet in the back alleyways of America's Koreatowns and Little Haitis--they live in America, but have given up on speaking better English, and thus have arranged their life in such a way they don't have to use it much. In contrast, I regularly meet people here in Taipei who speak excellent English, despite never having visited an English speaking country in their life. They've spent the hours needed to drill English into their heads and now enjoy the fruits of their labors.
So to sum all this up:
- Second language learning is for the most part a function of the amount of time you spend actively using the target language.
- How you study a language is less important than the amount of time you devote to actively using it.
- The only way to gain fluency in a second language faster is to increase the proportion of your time where you use the language in question.
The title of this post is a riff on a famous essay by David Moser titled "Why is Chinese So Damn Hard." It is a funny and well written essay, so if you have not read it before, go do so now. He offers a buffet of arguments, each contending that Chinese should be considered the most difficult language on Earth, and each is supported with a humorous anecdote or two. I do not think any of these arguments are incorrect, but taken together they offer a picture that is far more complicated than it needs to be. The real reason Chinese is so difficult is pretty simple, and it is best understood in terms of the language-study equation I laid out above.
That problem is the characters.
Now this is not the only hard thing about Chinese. As Moser points out, Chinese has few English cognates, and the cultural gap between Chinese speakers and English speakers is enormous (these are people who don't recognize references to Elvis, Darth Vader, or Santa Claus; most Westerners are just as blissfully unaware of the Chinese equivalents). But this is hardly unique to Chinese. I faced similar challenges when I learned to speak Khmer, but Khmer is still only considered a Level 3 language. Khmer and Chinese also both have relatively simple grammar systems--French conjugations are more complex than anything Khmer has to throw at you. On the balance Khmer grammar structures are probably slightly harder to to use than their Mandarin equivalents, if only because the pronoun system is so much more complicated than in Mandarin (and the consequences for using the wrong pronouns so dire), and Chinese syntax is usually a little less busy.
Unlike Khmer, Chinese is a tonal language, and this really is a significant difference. Tonality makes a language much harder to learn. But even tones do not automatically make a language one of the very hardest. Thai and Vietnamese, both tonal languages, are only considered Level 3 languages. The comparison with Vietnamese is particularly apt, for it is very much like Chinese. In addition to tones, the lion's share of Vietnamese words are of Chinese origin, both are an analytic languages that use SVO word order, neither has much in way of consonant clusters, and Vietnam shares most of China's cultural heritage. But Vietnamese takes several hundred class hours less to master. 
The difference is that Vietnamese no longer uses Chinese characters.
Characters are difficult for all the reasons Moser describes. But again, I think he over-complicates things. The real difficulty with learning Chinese characters is fairly simple: they must be memorized. All words you learn in a foreign language must be memorized, of course, but in Chinese you must memorize each word twice. You memorize a word once for its character, and once for its pronunciation. It is quite possible to memorize all of the characters without knowing the proper Mandarin pronunciation of any of them. This is how Koreans, Vietnamese, and Japanese have memorized Chinese characters for the last few millennia, and it is how books like Remembering the Traditional Hanzi teach you to do it today.
Memorizing characters is not particularly hard. It is just time consuming. This is what pushes Chinese into the 2,000+ class hour range. It is not because its grammar is more complex, phonology more difficult, or culture more alien than that of the 1,000 class hour languages. It is simply because the person learning Chinese must spend an obscene amount of time on characters. I estimate that I spend around 60% of my Chinese study time memorizing and reviewing characters. An informal survey of other intermediate and advanced learners I know revealed that they do the same. How much better would our Chinese be if we could take that time we spend on character study and use it practicing any other language skill? Characters literally double the amount of time it takes to master this language.
Which is why those who want to learn Chinese fastest skip them. This is what Benny Lewis did when he studied Mandarin in for his Fluent in 3 Months challenge. This is also what Mormon missionaries do when they come to serve here in Taiwan. Unless they studied the language before they served, the best can only write around 500 or so characters by the time they leave, though they can speak, very, very well. Which makes sense. They spend two years speaking the language day in and day out. They cram thousands of hours of speaking and listening into the first year and half of their service. By ignoring the characters altogether, they learn to speak more fluently than many foreigners who have been studying here for years, and in only a fraction of the time.
The downside to this is that they can't read anything, including the Bibles and Books of Mormon they carry around! But that is the trade off built in to any attempt to master a language that is so damn hard.
 I recently talked with an American FSO here in Taiwan who last was stationed in Hanoi. He reported that the FSI actually recognizes that Vietnamese is more difficult than most other Level 3 languages, and says that it would give the language a 3.5 rating were it possible to do so. In light of this, it is the only Level 3 language where the FSI has a language learning center in country so that students can take advantage of an immersive environment. All other Level 3 languages are taught in Washington DC.