I'm not sure, maybe 2-3 weeks at most. Nowadays if I'm not getting any pussy I'll usually rub one out at least every 2 days, sometimes more frequently.
The longest I've gone without sex was a good half year. Once you get past the initial phase it's not so bad; I've found that the craving for sex after long periods of celibacy (which people claim "drives them insane") is nowhere as bad as the initial withdrawals!
Right now I'm forced to live in a situation where I see my girlfriend maybe once to three times to month, on weekends, and obviously we spend a large portion of that time in the sack. Whenever we separate I'm uncontrollably horny and can't stop thinking about her! I never have time to get used to it, either, cause just before I feel like I've overcome the obsession temporarily we're at it again.