I think it's important to be consistent in what and when you feed. So my rabbits get vegetables every day. They normally get two different types. Greens are part of their staple veg diet - added to this is kale, brocolli, carrot, apple and other bits of fruit at less frequent intervals, mainly as a treat as they are high in suger. They get a bowl full... not weighed it though so would be difficult for me to say an exact quantity! They don't have any problems with weight of droppings so it must be about right. If they've been on the grass I reduce the amount of veg they get accordingly.
As other have said hay will form the main part of their diet which is then supplemented by pellets, veg and fruit.
Hannah