I had a girlfriend in college that was crazy for sex - I don't know what the the true requirements for being classified as nymphomaniac are, but she had to be close.
We had several memorable sex marathons that started in the middle of the night and went on until the morning. After I would cum a few times, I would be able to go for extremely long sessions. I have always been able to get hard easily and stay that way, but she put me to the test. I am sure we went at it for several hours one particular night.
So for me, probably around 3 hours, but that's after I had gotten off a few times...
My current girlfriend and I have great sex, but we never go for nearly as long as I just described.