On old tractors there was a built in hours meter, rather like the mileometer on cars. It worked by being turned by the cable on the rev counter, (tractors don't have speedometer)
I don't know if this crosses over to small two stroke petrol engines but it is pertinent. The "hours" is dependent on the revs rather than the actual physical time the motor is running. Tractor service intervals are therefore dependent upon the number of revolutions the engine has made rather than chronological time. If you apply the same to a trials bike the need to service it is dependent on whether it is running at quarter throttle round sections or revving it's nuts off jumping up a huge obstacle.
My gut feeling therefore is that the service schedule of any trials (or other competition bike for that matter) bike is really guesswork. A factory team with a big budget and a pro mechanic or two might well service the bike far more than it really needs, but at least it keeps the bike competitive. Me, I am rubbish and usually come last so being "competitive" isn't really an issue to be honest. Another benefit of being rubbish is never revving the bike up either. I don't think I've ever opened the throttle past half way.
So really the need to service and to replace items like pistons is really dependent on the individual use. Well IMHO anyway.