I've always felt once you go past 13-14 hours, the actors start losing energy and the results are just not worth it. And all the technicians start making mistakes, tempers start flaring... it's just not a great way to work. 10-12 hours is about right.
So much depends on the AD's preparation, the director making the right decisions early, rehearsing the actors and nailing down the script days or weeks before one frame of footage is shot... you do all those things, and the days can go really fast.
I can work almost any hours as long as I get a solid 8 hours of sleep the night before. But you have to build in travel time and prep time into all that, and the reality is, you're lucky to get 5 hours after all that.
Those 5-hour Energy Drinks only go so far...
