I guess there will never be a really satisfying answer to this question. We know that beyond time, effort/dedication, study technique, and personal ability all make a difference. One thing to bear in mind is that anybody who passes level 2 or 1 is at least a somewhat dedicated student. It doesn't mean that everyone who passes level 2 is fantastic at Japanese or is studying 6 hours a day, but based on my personal observation, the person who just spends a half hour or so looking through their textbooks once or twice a week, will never pass JLPT 2 even if they live in Japan for 10 years and can get by in social conversation.
I've known 2 people (one Chinese, one Korean) who have passed JLPT1 in a little over a year of study. This is generally considered an extremely impressive feat, but it's not unheard of. Obviously, they were living in Japan and studying full-time (meaning, they were full-time students of Japanese; I don't know how many hours a day they were studying).
Among the English-speaking discussions I've seen, 2 years is often mentioned as the "very impressive, but not impossible" yardstick -- 1 year to get to JLPT2, one year to go from there to JLPT1. So, pretty much what shneen said.
6 months for JLPT1? Well, I can't rule out that under the best possible conditions, someone somewhere might pull it off, but it's hard to imagine.

How long it takes to pass the JLPT depends largely on how disciplined we are in the art of Procrastination. Procrastination is an under-appreciated aspect of language study. Imagine how difficult it must have been to learn Japanese only five years ago for those who did not understand its potential?

In the last few years we have seen the emergence of SRS based learning sites such as Reviewing the Kanji, Anki and Smart.fm. Numerous media resources are readily available online now for listening practice and we can video chat free with people of any language on Skype. Native speakers enthusiastically offer support at Lang-8 for correcting writing errors. Rikichan allows one to read any Japanese website. There are various plug-ins available to convert kanji into kana for easy transition from novice to intermediate study.

Next year several new online darlings will appear for faster long-term absorption. Three years from now perhaps electronic devises will be attached to our brains and ears at night while we sleep that teach us subconsciously with no effort what so ever. A new pill will emerge with no side effects that help us focus. Though, true breakthroughs will be in areas we can not imagine now.

If we are disciplined in our use of Procrastination, the longer we put off study, the faster we will one day be able to become fluent and/or pass JLPT.

boshisamaya: So the best way to study Japanese is not to study it? Spoken like a true zen master!

To those who say that the JLPT 1 is hard for native speakers, I say that it is not. At least, that's what one Japanese told me. Also look at this post by Aijin:

Quote:I'll second that. I am not sure where that myth came from, but I've heard it a few times as well. I have never taken any of the JLPT tests, since that'd be kinda' silly, but I did check the sample test the first time I was told that by an American student. I can see where it'd be very difficult for a foreigner learning the language, but I can't imagine any natives having trouble with it! It's very easy compared to language tests encountered in the Japanese school system, especially the higher grades.

And yes, this was about １級.

So just remember... The sooner you fall behind, the more time you will have to catch up