Plan how long you want to work each horse and wear a watch.
Figure out how much time you have and divide it by how many horses you have to ride that day. Time each ride and try not to spend too much on one horse :)
Plan out my barn time and things I haft to do then make a schedule and follow it so I can be done
During holidays, it's easy to find an other rider to help me, and we ride at the same time. If I don't, I ride one, every day. For example, I ride one on the Sunday, and the other one is lunged or has a rest :)
I have personally two, every time. During the week I take one in the woods and the other in the arena, or I lunge both, or one working, the other having a rest. It takes me around 3 hours every day. And for the week-end, I'm at the stable around 8.30 a.m, I'm looking after my horses, usually jump or competition, if not trotting, it takes me the half of the day but that's a choice!