Generally, the previous owner will tell you what feed the horse eats, as well as any supplements they get. It's recommended to feed 2x a day approximately 12 hours apart to help keep your horse regular internally, especially if turnout isn't available 24/7.
I feed at about 7 am and 5:30 pm. At each feeding I give 2 slices of hay and 12% pellet grain. They get one hour of turnout to an alfalfa field. (I'm slowly increasing their turnout and decreasing hay)