Our three year olds start off with 30 mins long rein a few times a week, then once backed fully its 30 mins about twice a week. The more the horse develops, the more it gets ridden. However it obviously varies what the work your doing with them is and what you eventually want them to be doing. If shes just walking on roads I cant see this as being awful, 90 mins on the gallop however would be a different story! I dont think there are any set rules, just whatever feels right for the horse. Some progress faster, some need more development before they can cope with that kind of work.