It does definitely depend on age - I'm Australian-born Chinese, and I entered kindergarten barely ever having spoken English, and I was as fluent as the other kids in a couple of months (don't know how many exactly). However, that of course is a very limited level of fluency - the level of a 5-year old. If you want the fluency of a twelve-year old (which is what the vocabulary of newspapers are supposed to represent), I would guess that that would take a lot lot longer. If you mean fluency of an educated adult, I'm pretty sure that's impossible to obtain without living somewhere where the language is spoken and written all around you for quite a number of years. Someone may think they're fluent, but bring up some science term or some sport term or just some commonly-known word from a discipline or activity not commonly spoken about, and you will see that most people who think they are fluent lack vocabulary beyond what is used in common conversation and perhaps their field of study/interest. However, if you define fluent as in able to have a full-speed conversation, than that is probably achievable in a year (e.g. I generally claim to be able to speak Cantonese fluently, even though I essentially can't as my vocab is exxtreeeemely limited; because I speak it at home and thus can keep up with the fastest Cantonese speaker and respond in fashion, as long as they use common day-to-day words that I know :P)