I keep a good quality mixed grass round bale out for our three horses all the time, and don't add another until it's almost all gone. It's free choice and they can eat all they want.
We also feed a one quart 50/50 mix of strategy and crimped oats twice a day.
I've wrote down each and every time I put a roll out, and to my surprise, I fed more round bales in August when it was dry and grass wasn't growing.
Spring time is when the hay lasted the longest.
I don't know if there is a correct answer, as how much a horse eats depends on several things, how much work they do being an important aspect. How much they're stalled etc.
I'm of the opinion that a pleasure horse that's only ridden on the weekends needs very little grain or feed.
We keep square bales too, and feed those when we stall the horses after riding. Our horses are stalled during the day in the summer, and at night during the winter.
By the way, I've put out 16 rolls this calender year.
I don't think you can feed to much mixed grass hay, only to little.