I know that daily riding and excercise is good but is there a point where it's just too much riding for the horse? What if if I'm just riding, nothing intense.

It would depend on how fit the horse is. Just like how a marathon might be too much for the average person and all day ride might be too much for your horse. You can tell based on how tired your horse is when its time to stop.

Usually, I've found that if they are still having fun at it, and not acting worn-out, then it's still a good work load. It also depends on the quality of food they have and if they've had a good supply of water throughout the day. They need to keep hydrated like humans, and will be able to handle more exercise if fed and watered periodically.

I was always told that horses, if you are in exercise for a full day, will need break times. Kind of like when people need breaks from being on the computer for too long. Does the break time depend when timed on how long you have been going for or is there a set rule (like the eyes and computers, 10 minutes per each hour).

It depends on the breed and what the work is. Thoroughbred racers run for less than ten minutes a day, at most they would have a sprint at full pace and a gallop at half pace. Standardbreds do more work when they're training for harness races, showjumpers and dressage horses do even more work in a day....... it really does just depend.

I think it depends on what sort riding you are doing. If you are training for competitions or what not, then you should limit the time each day, how long would be depending on what you are doing. If you are out for leisurely pleasure riding, then most horses can go all day, with little breaks for meals and water.