back in college I worked at a saddlehorse farm, they had a old school horse that was used to 30 minute rides.... if the student had a hour's class, they had to dismount and get back on as Pete would stop until the next session began.
His internal clock was correct to the minute
My school horses live out in pastures with run in sheds. They usually have at least two days off a week. They do 1 or 2 lessons a day, occasionally one horse may have to do 3, but that is rare and I make sure it's not always the same horse. Lessons are not very hard, mostly walk trot, some cantering. I have a few students who do more cantering and some jumping, usually in that case that horse only does that one lesson that day.
at my barn, depending on the horse (and the weather), i think it's maximum three lessons a day, no more than two back to back. some horses (young, healthy, energetic) may go more (usually with very experienced riders), others (elderly, creaky, coming off layup) may go less. there are periodic no-jumping weeks to give them all a break; most of the instructors do trail rides several times a year to get out of the arena. horses are given several weeks off on a case by case basis if they need it.
I lease my horse (mule actually!) to a local barn where he is used in beginner lessons. He works 4-5 days a week up to 60 minutes a day, and sometimes on Saturdays he does 2 lessons.
I provide chiro and soon hock injections in order to keep him working comfortably. Saddle fit is also very important to keep an eye on, esp. with beginner riders flopping around. He also wears a sheepskin half pad under his saddle for extra padding. He is out on pasture at night.