I weigh it for each horse, they are in separate turnouts.
Lily (pony) is getting about 3% of her body weight per day + about 3 lbs for "waste." It's about 30 lbs a day. She would eat more than that if I allowed her, but then she'd probably get too fat to walk!
When it's extra cold (below zero F)/snowy/windy I give her about 5-8 lbs of alfalfa/grass mix.
Soda (QH) gets between 2-3% of his body weight per day + about 3 lbs for "waste." He ends up getting less than Lily because he doesn't eat all of it, so he's basically adlib. Again when it's extra cold/snowy/windy he gets 5-8 lbs of alfalfa/grass mix. He always cleans that up first!
I have 3 types of hay I'm feeding too. A mature meadow hay, I feed this at night and sometimes will add a flake or two to the daytime feeding. It provides a higher fiber content and more warmth in digestion. A really nice leafy grass hay, this is fed during the day when they don't need as much warmth. Then the alfalfa/grass mix, it's a good leafy grass and a younger leafy alfalfa, the mixture is about 40-60% alfalfa.
It gets a little complicated at times, but I'm overly anal about my horses....