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1) The best way to learn a language, after initialisation [read: knowing 500-1000 words in that language and some basic grammar] is to read serious stuff in it. Every new book [300 pages] gives you 300-1000 new words, and their contexts, and teach you to think in that language.
2) A native language speaker will have knowledge of 15000-45000 words in their language. But to be thoroughly functional, you only need ~5000 words. Reading ~10 300 page books will give you that capability. Assuming that you read 2 books/year, you will be fluent in 5 years.
3) This brings you to advanced fluency levels, with only some very specific topics beyond you. At the same time, you should start writing in the language. Maybe just summaries of what you read. Writing short essays on what you want to express will get you to the next level.
4) At the end of 5 years, you will be very comfortable reading and writing in the language of your choice, to the point that you can read whatever you want and write mature, well-written essays/articles that can be published in newspapers/magazines.
5) After this, you won't forget this language ever. However, this brings you to around high-school pass out levels of fluency. You are still not a master of the language, but you are very functional. Continue reading and writing, and you will get to native fluency in 10 years.
6) In reality, even committed polyglots are rarely at native fluency levels in many languages. I have rarely known people who are at *native* fluency levels in more than 5 languages, even though they are at advanced fluency levels in 10-15 languages. There is a reason for this.
7) Once you get to ~5000-10000 word mark [depending on the language], your progress will become ever slower. The law of diminishing returns begins to strike. Each new book you read will give you fewer new words, contexts, etc, even if your books are specifically chosen.
10) If the first 10 books 300 page books can give you 5000 words, the next 5000 words are likely to take 50 books. But passively, the more you read, the more you will be improving the intangibles like reading speed, comfort with expressions, your thinking in the language, etc.
9) What I have written above is true for Indic, European, Persian, Tibetan and SE Asian languages. What I have said may not be true, though, for the East Asian languages like Chinese, Korean and Japanese, which work in a completely different way.
