My horses are fed with hay 2 times a day: in the middle of the morning and in the middle of the afternoon. I also supplement them with fiber. They live in a barn. If one of the horses needs to loose a little bit of weight, we always diminish the amount of grains and never of hay :)
I know a horse should be fed several small meals throughout the day. And a 1000 lb horse should be fed 18 lb of dry matter each day. If you are feeding 100% hay and your hay contains 90% DM (or 10% moisture), your 1000 lb horse should be fed 20 lb of hay (18 lb DM/0.9) straight from the bale each day. So they depend to the activity of your horse.
I give hay five times in a day, the quantity is different, for example I give a bigger one for the night.