Our horses rarely get any grain or other concentrated feed, They have access to pasture nearly constantly and get hay when the pasture is recovering. I have found that I don't like the attitudes my horses get when they are fed grain. They are much nicer animals when they have to find their own food.
I would recommend feeding as much good quality hay as they want when they are not on pasture. This not only provides nutrition, but also activity so that they don't get as bored and develop bad habits. Monitor their weight as you begin working them (a height/weight tape is a great tool) and start to add grain if the begin losing weight.