Is there a KPI to measure average hours flown?
I want to set a KPI to measure average hours flown.

You might want to check block hour:


  (aviation) The time from the moment the aircraft door closes at departure of a revenue flight until the moment the aircraft door opens at the arrival gate following its landing. Block hours are the industry-standard measure of aircraft utilization.


(Emphasis mine.)

The Wikipedia article airline cost glossary lists very common terms and KPI's.

One of those terms that could also be applicable to your case (revenue passenger miles) was asked about here: Which aircraft type flies the most annual passenger-miles?