The longest I've ever gone is about a week, when I went on holiday with some friends. Everyone else wanted an 'wank-abstinence' rule, I'd have prefered not to have it, I'm not really bothered what other people do in the dark, even in a shared room, but I didn't want to appear over-eager to wank off in my friends' presence. I decided to take it as a challenge, because before that the longest I've ever gone without is 2-3 days, I nearly gave in, but I stopped mid-wank (in the shower).
When I got home I had a brilliant wank, I was really backed up, so it was definitely worth it! However, I've never managed to achieve such 'heights' since then, I just don't have to will.