Ask the person that sold the horse to you
My horse gets hay and grain 3 times a day.
At 7am, 12pm, and 5pm
It all depends on the the horse, as some may run healthily off of grass, but some may need to be fed 2-3 times a day. It all depends on the individual horse. :)
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)