Idk, probably 16 hours straight. When I was in a junior college right after high school I got a job at a plant making PVC pipe, and we had rotating shifts. That was years ago but I want to say it was like eight days on, one day off, repeated for three or four weeks then you'd get almost an entire week off. That was crazy because it was really good money, I bought a good used car after working there for one summer, but they let us volunteer for overtime and we'd end up working way, way too much. Sixteen and eighteen hour days or more were not uncommon. Paychecks were really nice though. But I remember coming home after one of those eight day runs ended, and just sleeping forever.
I actually probably have slept for longer than that, probably close to 24 hours before. But that was in my early twenties, during my drug days, when I had a really bad coke habit, and tried pretty much every drug known to man (with the exception of heroin). I started out doing bumps of coke at a party and then got introduced to a friend who knew a dealer, then I got to know the dealer, then I had two dealers, because yeah, one wasn't enough, ffs. I went from doing bumps to doing grams, to eight balls, and half ounces, and then ounces, and then buying even more than that a few times. Thank God I don't do that **** anymore. But yeah my friends and I would do that and / or speed, and drink....and the coke and speed let you party for what seemed like days. And then we'd crash, and sleep for forever. I'm very sure I've slept for more than 24 hours before. Like I said I'm so, so glad I don't put that stuff in my body anymore.