It all depends on how in shape your horses are, I live in florida and it's pretty humid and hot so we only ride in the mornings or evenings.
But when I ride my horse bc he's pretty in shape I ride 6 days a week for about 2 hrs or so each ride and he doesnt even sweat! Or get tired. We were getting ready for some really tough shows last year and I think one ride we had was over 4hrs long and he didnt sweat at all so it all depends, he doesnt even start breathing hard and we work on collection, extension, dressage movements, jumping and trail rides.
I know you guys prob think i'm crazy, but my horse loves to be ridden and to work, he actually with collect himself if i'm taking to long and go into a working trot in a frame and everything lol!