Actually it really depends on how much effort you want to put to studying the language. I've heard of people with no background, moving to Japan and passing JLPT2 within a year. For myself, I took the JLPT2 exam after 2.5 years, 2 years of which were spent attending class only once a week, but infused with trying to read mangas, novels and teaching myself JLPT2 grammar.
But JLPT isn't a really good indicator of ability in the language to be honest. Try reading stuff like the Japanese 2ch forums and you'll see where you really stand in terms of understanding colloqualism. That's the real challenge I've been facing; speaking is another major problem since I don't have any Japanese friends.
Just to show how it all boils down to effort (let's just exclude the natural linguists): I personally spoke to a person who never took Japanese classes, never stayed in Japan but picked up the language mostly by learning how to read hiragana/katakana and then learning the language through anime, manga, progressing to novels alone (granted this person's native language is Chinese so kanji wasn't much of an issue). And this person's understanding of the Japanese language is really impressive: able to write letters; can understand 90% of Japanese seiyuu radio. His only gripe is because he never attended formal Japanese classes, he doesn't really understand grammar in a logical sense like how they teach it in class but from an intuitive sense, and his wish is to one day find time to sit through a beginner's course to learn grammar the 'proper' way!
