I think seven, although it's usually 2-3 times a day. Maybe one of these days I'll try to break it if I have the time and no one's around. Although like others said, once you get past six or so it becomes a bit more painful (not pain painful but more like sore painful).
5 is my record. For the life of me I have a hard time seeing how any guy can do it more times than that.
And I'm not saying that from a moral standpoint or anything...believe me, if I could do it more times than that and enjoy it, I'd go for it! But it's not just the soreness (and I mean muscle soreness, not skin soreness), but also just the pure exhaustion of it. Like someone else said, after the third time, even if you space them out, it's like you're just pushing yourself and it's more of a chore than an enjoyable experience.
I'm not very well versed in sexual biology/psychology, but I've always found it interesting to compare you're mental interest in arousing material with your physical limitations. It's like the more physically worn out you get, the less you mentally even want to think about things that arouse you. I guess when your body doesn't respond to your mind, there's no fun in the mental pictures.