I think 18 hours when I worked as a PA on a movie set. But some of that time included the transportation via bus to and from the set, during which I slept. I got paid for that time, though. It definitely didn't seem that bad because there is A LOT of downtime on movie sets.

I do occasionally work 14 hour days for my current job at a caterer. That's rough because I'm on my feet the whole time, usually doing a lot of heavy lifting or baking away in the hot kitchen.