Riding school work is hard for a horse - they need to adjust quickly, many riders are not very capable and unbalanced. On the other hand, there's no chance of one mistake having long-term effects- every rider is bad in _different_ ways, so if you keep the horse tuned up, they can live forever. In my experience, two hours a day is a long-term sustainable workload for a riding school horse - they can occasionally do more, but no school I've known that regularly used their horses for three or four hours managed to keep those horses healthy for long. (Hacking/trekking is different, but an hour in the school counts for two of those.)

It sounds as if you've found a great school. I'm a great believer in riding schools - think what would happen if all of those people bought their own...