45. I am 25 now, so 20 years.
Several reasons here. First, I LOVE my work. I can't imagine much that I'd rather be doing with the bulk of my day than what I do now. That may change over time, but in my estimate now, I look forward to a lucrative career making the world a little bit better.
Second main reason, I want to save up a lot and don't expect to make more than, roughly, 80k at the max, with 60k being the average. I have more expenses than the standard bear, so I want a nice fat nest egg to sit on when I roost.
I figure, if I can get into a 60k/year job before I'm 26, retiring by 45 will be no problem at all.