I too think it's a great question and I'd love to know the answer. 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. We've received a good answer to the second part of the question.

I'd suggest looking at what FedEx does--they can run planes close to round the clock if they have to: unlike passengers, packages don't care when they file. I just tried to find data on equipment usage at FDX, but haven't found it. Anyone got ideas on usage of FedEx equipment?