What happens if you leave the APU on?

Excuse me, could you elaborate on what happens if one were to leave the APU, or Auxiliary Power Unit, turned on? Is there a potential risk or hazard associated with this? Additionally, how might it impact the overall functionality or efficiency of the system it's connected to? Is there a recommended timeframe for when the APU should be turned off to ensure optimal performance and prevent any unnecessary wear and tear? Thank you for your time and consideration.