Thank you ericferret, but I do know what time in service means, and it's not relevant here.
The FARs sections concerning an approved inspection program (§91.1109,§125.247,§135.419 take your pick) agree it must include
(2) A schedule for the performance of inspections that must be performed under the program, expressed in terms of the time in service, calendar time, number of system operations, or any combination of these.
So if "hours of operation" is going to be a factor in an approved program it must be equivalent to one of the above.