How long a horse should be exercised depends entirely upon the horse's fitness level and the type of riding. A horse that has not been ridden regularly for a few years should be exercised very little at first. A horse out of work should be slowly brought back into fitness by extending exercise periods. For example the first week or two you should mostly walk with some trotting in short bursts. Over time increase the duration of time spent trotting and after several weeks mostly trotting begin canter work. A horse out of would show be exercised maybe only 15-30 minutes a day, while a horse at the peak of fitness may need an hour of arena work and a mile or two gallop to maintain physical condition. How long a horse should be exercised will vary with each individual horse.