I think an hour out on pasture every other day or at least twice a week. Give a horse a little extra. Purina Even makes a 5 pound bag you can pour into your horses bucket to supplement nature forage if you can't get them on pasture.

Something in Gods great design helps that nature graze. It sure doesn't take em long to stick there nose to the earthworms and start ripping when they have been boxed for awhile.