For me, it really varies with each snake. Some I can tell where they are in their Eating-Digestion-Hunger cycle simply by watching their actions at night. Others I actually have to lift the hides every couple days just to make sure they're still in the tub.
Then I have the one I refer to as The Furniture Mover...About every three days this kid flips, moves, or knocks over everything in his tub except the water bowl, and as soon as he gets big enough to move it, that's probably going in the mix as well.
If it's a night where they feel like coming out of the hide, I would say they average about 1-3 hours of activity. But this will always vary by snake.