Hay is the most important thing. She should be getting free choice, or close to it. After that, the amount of feed just depends on her metabolism and how much work you're doing.
My horses only get about 1-5 lbs of feed a day, plus 1/2 - 1 cup of flax (linseed) meal, with whatever supplements they require. My easy keepers or pasture pets only get 1 lb of chopped alfalfa hay (lucerne chaff) to mix their supplements and vitamins in. The horse that gets the most feed is a slight "hard keeper" and gets worked 2-4 days a week. He eats 1/2 lb of chopped hay, 3 lbs of alfalfa pellets, and 1 lb of oats with 1 cup of flax. That's really not that much food, considering. But, all of my horses have some grazing plus free choice hay. They get their "feed" just once a day.