Well, according to our vet, there is not really a limit to how many greens. The minimum seems to be about 1 heaping cup per 4 lbs body weight, but they can eat as much as a heap the size of the bunny per day! When they get that much, though, they don't get extra pellets, just hay.
Our Hazel can definately eat almost her size of greens in a day, she gets 2 full baskets of greens, 1 in the morning and 1 in the evening. She also has access to hay, but she doesn't seem to like it much. We're trying to get her to eat more hay though, so we dried some mint and basil from our garden for her, and she likes those. Very spoiled bunny
Our Chocolate is starting to eat more greens now, though as she was a shelter bunny and used to pellets and hay, she doesn't eat as many greens yet and still gets pellets at night, and has hay aswell.
You do have to be carefull though and make sure your bunny tolerates vegetables, and definately very gradually lower the amount of pellets while adding more greens.