"Hours run" is a highly arbitrary way of assessing the likley wear and tear on an angine in any case. With cars we do it by miles rather than hours. You could just as easily make out a case for doing it by "total number of revolutions" run. Some jets, e.g. I believe the Allison in the Bell Jet Ranger engine, are lifed by the number of starts. For most of the piston engines we fly, cold starts are the time when most engine wear takes place, so an aeroplane that is flying most of the day would usually have a far longer time to actual failure (in hours run) than one that gets flown twice a week. And the limiting factor for a lot of light aeroplane engines is internal corrosion that takes place when they are not running at all. So running "on condition" actually makes a lot more sense than running to an arbitrary hours limit, providing it is being properly inspected. If your group looks after the aeroplane, changes the oil often, pre-heats it in the winter and flies it regularly it might go on for a very long time