Well, fluency is quite a relative term. I guess I was able to understand most Japanese texts after about 5 years, but still not quite everything. My current level probably took me around 8-10 years to achieve. Now, I very rarely come across words that I don't know. However, I'm not sure whether I'm that representative because I studied Japanese on and off; sometimes absolutely nothing for a year, but the next year I spent my freetime almost exlusively on Japanese. I know a few guys who passed JLPT N1 after only 2-3 years (though the JLPT is not really that good a measure), so it all depends on your dedication, I guess.
You also have to remember that the study of a foreign language (or even your mother tongue) never really stops.