It depends on your goals, your horse and whether or not you ride competively. My horse and I show all the time so I have to make sure he is well conditioned to heavy work, he gets ridden 6 times a week and jumped twice :)
For me, it really depends on the horse first, and on the rider and his/her objectives. Anyway, everyday is too much for me, but if your horse doesn't live in the field he has to go out of the stable everyday, so you have to vary activities and intensity.
My horse lives in the field, he works three times a week in general, and the rest of the time he stays in the field or we go for hack 🙂