It varies from airport to airport and operator to operator.
An example, Stockholm are very insistant that APU use is kept to a minimum, so much so that when 787s began to land there the airlines had to negotiate a special exemption to allow APU to be used throughout the turnround.
The 787 APU at the time had a 'bowed rotor' problem which meant if it was shut down it couldn't be restarted for a couple of hours.
B787 APU Heating Issue
Operators have discovered that after the APS5000 APU is shutdown with the inlet door closed after landing, heat continues to build up in the tail compartment. After some 20 min. this causes the rotor shaft to bend or ‘bow’, and the shaft takes up to two hours to straighten back out.
As many 787 operators, including
All Nippon Airways (ANA), have flown the aircraft on shorter routes with reduced turnaround times, this has resulted in restrictions on when the APU can be restarted. An advisory bulletin from
United Airlines says that if an APU start is attempted 20-120 min. after shutdown with the inlet door closed, the “bowed rotor shaft can cause turbine rub and significant damage.”
Stockholm does offer very good ground power/AC facilities, unlike some UK regional airports who struggle to get two GPUs on stand and have no AC.
My local airport has no restriction on APU use. Which is just as well in midsummer.
Some airlines demand that their pilots do not start the APU until absolutely necessary (in some cases just 5 minutes before STD). This is not necessarily due to fuel use but APU maintenance hours. The less you use it the cheaper it is to operate especially if you are on an 'on wing' maintenance contract with the supplier.
They do have a get-out of course and it will be in their company docs that if cabin temp becomes uncomfortable then the APU may be used.