Ultimately, I think it depends on the individual horse and what exactly you want to achieve with them.
Being I primarily barrel race, I need my horses to be in top physical condition, and have plenty of "air in their lungs".
In the spring when I'm trying to get the horses back into shape after having the winter off, I definately try to ride every single day. I start their workout program gradually and build on it. Once they are in shape, a typical workout on the trails is 1/2 mile warm-up at the walk, followed by 4 miles (or more) of alternating and randomizing trotting and loping, followed by 1 mile cool-down at a walk.
Once or twice a week depending on our hauling schedule, I will breeze them (let them run full-speed full-blast). They really tend to get into running shape when we start competing and hauling on the weekends.
Once or twice a week we will do exercises in the arena, whether it's reining, jumping, or slow work on the barrel pattern. Usually the sessions are around an hour, but could be more or less depending on what I wanted to accomplish that day. If they "get it" right away, then we'll stop and be done, even if it only took 20 minutes.
Red tends to fire harder in his runs when I ride him every other day when he is good and legged-up and in the middle of the season. Shotgun is still fairly young so I don't expect him to perform at his top yet; but I suspect he will also be an every other day horse.
Red has calmed down a lot the last couple years. Years before, I would definately need to ride him every day to keep a more level head on his shoulders.
My horses are turned out 24/7 on pasture, so they are allowed to move freely throughout the day.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.