"I guess there's the technical answer--how long can you run these things--and the more practical answer: how long do airlines typically run these carriers."

That sums it up.....

Technically, an airplane can fly as much in 24 hours as it's range and minimum turn-around time at the stops will allow, at least over the short term and assuming nothing breaks that can't be fixed on the turn-around. This is where the long-range airplanes flying long-range routes have a utilization advantage because they can spend more time airborne relative to sitting on the ground. In theory, you could see utilization as high as 20-22 hours per 24.

Of course, in practice you almost never see maximum utilization because of slot restrictions, preferred departure times, etc.

For shorter range aircraft - 737's, A320's, and the like - most flying stops between roughly midnight and 6 am except for transcons, so both maximum (due to the shorter range) and actual utilization are much lower. I think Jetblue had the highest utilization for passenger carriers before getting the 190's at about 13 hours per day, and that was with a high percentage of transcons and JFK to Florida flying. As someone else said, averaging 11 hours a day utilization on their narrow-body fleet is about the best most airlines can hope for.

Jim