Depends... my tolerance fluctuates widely... I like to take breaks to get my tolerance down... at that point, on a 2+ week sober head, I'd say I take 1-2 hits and I'm good for hours. In fact, if I go much farther than that I start to trip out and lose touch with reality and my senses. Which, well, I do that on purpose sometimes, but it can be pretty disorienting if I keep toking on it because I remember what it's like to have high tolerance and I want to be able to just puff on it. I have to ease into it, though, or it WILL kick my ass.
On the other end of the spectrum, I can smoke bong load after bong load after bong load and never really get anywhere near the plateau I described above. It's all about tolerance.