Twenty or (more likely thirty) more years ago, APUs were frequently rum from shortly after touch down to after engine start. Today it may not even be started after touch down, leave one engine at idle till external is connected. If cooling or heating is needed a ground unit will be attached.
Certain airports have rules on APU usage, usually for noise. APU started just prior to engine start and shut down shortly after the last engine is started.
On certain aircraft the APU may be left running to provide heating, cooling if a pac's off performance is needed.
For long haul operations say two sections in 24 hours, maybe 30 minutes of APU. Short haul without external start carts, maybe something less than 90 minutes.
A lot of early 727-100 came from Boeing (United) without an APU. Cannot think of one after that, that did not have an APU.