Longest? Around 3 hours. Most of my presentations end up in the 45 minute range, and the vast majority are research-related.
My advice: NEVER REHEARSE. However, this style certainly won't work for everybody. I have a very comfortable style of talking, and I usually think ahead while I'm actually presenting. If I rehearse, I find myself trying to remember cues and such, which makes it feel more forced. And for God's sake, never read off the damn slides.
Edit: When I refer to rehearsing, it means a few things. For one, I never look at the slides after I've made them and are happy with the outcome. Two, because of this, I certainly never go through them and make notes of things to say. Because I'm familiar with the subject matter, I shouldn't feel the need to have anything outside of a mental template.