Hay should be the basis of the diet. Use a good grass-based hay, like Timothy hay. They should have fresh hay available constantly, since it helps prevent all sorts of digestive issues.
Pellets are more of a vitamin/calorie supplement. Up to 1/2 cup a day per bunny is a good amount. Reduce that amount if you notice he is eating less hay, or if you notice any excessive decals (mushy, smelly poop).
Veggies are the third major part of the diet. The amount you can give depends a lot on the bunny and what you’re feeding. As long as they eat their hay and their poop looks healthy, there isn’t really a limit. One thing to watch out for is gas-causing veggies like broccoli or cabbage – introduce those very slowly and don’t make them the major part of the salad. Some good staples include any leafy lettuce besides iceberg, kale, parsley, cilantro, carrot tops, and collard greens.
Treats depend a lot on how much sugar content there is in the fruit/veggie. For reference, my bunnies usually get about a 2-inch piece of carrot per day and no more than ~3 raisins OR a 1-inch slice of banana OR a small slice of apple/peach.
I generally feed mine twice a day, once for veggies and once for pellets/hay. This helps keep them from ignoring the veggies in favor of pellets.