>how long can you run these things

The US Air Force has a pair of VC-25As (numbers 28000 and 29000) that are specialized Boeing 747-400s frequently referred to as "Air Force One".

These planes are capable of mid-air refueling and I recall reading that, with the aforementioned mid-air refueling, they can 'stay up forever.' I don't know if they have special engines, or if this is just the nature of modern aircraft, with huge duty-cycles on the engines and other moving parts.

Cheers,
Geoff Glave
Vancouver, Canada