The origional question was 'how long can the APU be run, and why does APU air delivery seem weaker than engine delivery'?
Without splitting hairs about aircraft types and operator/airport requirements, the answer(s) should be:
indefinitely, as long as there is fuel to feed it, and TBO's on turbine, compressor, and other critical components are not exceeded;
air delivery will seem weaker in most cases because the APU compressor is much smaller than the engine compressor, the amount of air available for bleeding from the APU is less than engine, the delivery pressure from the APU is lower than the engine
My $0.02 worth .....