Well I would hazard a guess from what has been said, that if the defender has a strong defences, stone walls, trebs etc then it could well go well over the hour. I mean if a seige is a seige then it doesnt necessarily have to go immediately to battle, I would have thought you could hold under seige for extended periods until the defender either goes to battle, concedes or runs out of resources. If your defences are strong then you can hand over to AI and go off to work without too much worry.
Perhaps you could hand over to AI mid-battle too if you have too, who knows?
Likewise if your houses are made of straw so to speak and your force of 1000 faces a force of 10000 then the battle is likely to last only a few minutes.
Just my thoughts...