Do you remember the 10 000 hours principle, popularised in Malcolm Gladwel's Outliers? Well, it comes from research by Swedish-American scientist, Anders Ericsson. I talked with him. He says that there is no precise, scientific definition of "fluency", so you actually cannot construct an experiment that could determine which method works best.

In short: to reach moderate fluency at B2/C1 level, learning any language, would require a couple of hours every day for about 3 years. But there is -NO- optimal method!

I have put a considerable amount of effort to research this question as a semi-professional (currently studying applied linguistics) and for my own private use.

I've reached fluency in English (and to a lesser extent in Hebrew) as a second language. I've also learned and sometimes use Spanish (learned at a university), German (high school, I live in Germany now), Danish (university), French (high school), Ukrainian (university), Italian, Latin (high school), Classical Greek and Aramaic.

People studying full time Chinese, Arabic or any other language get their BA in 3 years and are quite fluent. It often requires about 10 hours a day of work (classes, reading, drills). It's hard. No short cuts.

On the other hand, however, I'd say that you need about 50 verbs and about 200 other words with almost no grammar to communicate. Where I work I speak Portuguese (a language I don't know!) German and Spanish with a girl from Portugal who speaks only Portuguese. The notion of "learning" a language is a construct of our education system. Grammar is almost useless is day to day communication. You only need both sides to wish to communicate, and there has to be no superiority and inferiority in the relationship. Somehow a natural "pidgin" grammar emerges spontaneously - you may not know past tense, but then you say simply "yesterday" + infinitive and it works perfectly well. The more I talk with Amalia the more Portuguese I get. And then I use it with two other friends from Brazil. It simply works - with no formal training, courses, textbooks. In class you are focused on correctness, not on getting your message across, and you are graded for correctness. This creates stress, confusion, doubt in your abilities.

My Portuguese, however, would not be good enough to get a job in Portugal. And my English, by the way, which I use with ease, would most probably be not enough to work as a journalist or in a radio station, although I read and listen to English between 5 to 10 hours every day.

What the research about language learning teach us? Almost NOTHING! It only confirms common truths about what helps: immersion, having no stress, living in the country, being self-reflective about your methods, good resources, practice, reading, radio, tv, vocab drills etc.

I talked with prof. Anders Ericsson about why is it that 40 years of serious systematic research has not produced ANY conclusions. You might have heard about Stephen Krashen and his "silent period" and "natural acquisition method", in short: adults learn just like children. This method is very popular among polyglot YouTubers such as the popular Steve Kaufman[1]. The most important principle of this method is that you don't learn grammar at all. The research on second language acquisition is NOT CONCLUSIVE! I believe in science (the same science that builds transistors smaller than visible light waves) and apparently Krashen's theory has not been confirmed or rejected which means that we still have no clue what works and what doesn't. I've spent tens or maybe even hundreds of hours reading about Krashen and I am only frustrated. Language research is tricky, there are dragons, don't go there.

I spent over a year on scholarship studying Hebrew, I was very methodical about it, I made beautiful statistics, graphs, precisely measuring everything for 12 months and my conclusion is that: leaning a language is freakingly difficult, requires inhumane tons of hours, and that brute force works (Anki drills). I had excellent conditions, money for free, a room, teachers, no family, no concerns. I can now (slowly) read academic papers and watch movies, but I just cannot imagine anyone (not super smart) learning any language having a (intellectually demanding) day job and kids, and reaching fluency on a graduate level.

I am about to start leaning Arabic and I feel I will die trying (I'm 30). With just about 3-4 hours a week I expect to be able to read Judeo-Arabic in 15 years.

Resources (in fairly random order):

* Julia Herschensohn, Martha Young-Scholten (ed.), Second Language Acquisition (The Cambridge Handbook), Cambridge University Press, 2013.

* Carol Griffiths (ed.) Lessons from Good Language Learners, Cambridge University Press, 2008.

* Christine Pearson Casanave, Controversies in Second Language Writing, Dilemmas and Decisions in Research and Instruction, University of Michigan, 2004.

* John W. Schwieter (ed.) Innovative Research and Practices in Second Language Acquisition and Bilingualism, John Benjamins Publishing, 2013.

* Anders Ericsson, Robert Pool, Peak, secrets from the new science of Expertise, 2016. (interesting but not strictly scholarly)

* Stephen D. Krashen, Principles and Practice in Second Language Acquisition, University of Southern California, 1982.

[1] https://www.youtube.com/user/lingosteve