ours are always started on 20 mins then its 5 min increase a week providing all goes well. usually start with a week on walk, then introduce trot. Then its all about the indiidual horse canter wise
we usually just let the horses tell us what their up to. Driving si slightly different pressures tho, it normally starts a week of walking then trot in short bursts increasing duration for the next few weeks then faster work as and when the horse requires it.