I feed my adult Holland Lops that are not currently breeding 1/2 cup a day. We are currently feeding to a 17% protein feed, however I recommend that you feed a 16% feed for animals that are not breeding. Why? Feeding too high a protein can make your bunny overweight. And high protein for the buns that don’t need it has been linked to kidney problems.
So how do you know if you are feeding your rabbit enough? By their behavior!

If they are frantically eating once you feed them, try upping their food.
If they show little to no interest in their food, try giving them less.
If they are interested in their food, but aren’t acting like they are starving, they are being fed just right!