It depends on what you want , I do not feed veggies, i do not believe it is necessary and it has caused me to lose a rabbit or two in the past, never again for me. If you want to feed veggies thats your choice
1. How much fruit/veggies a day? 1 cup per 2 pounds throughout the day. ( If your rabbit is 6 pounds it gets 3 cups)
2. How many times a week? If your making it part of the main diet, everyday. if not than you only have to give a little bit as a treat (Mine get it once every 2-3 months, they will get herbs next year in the summer though, mine also go out daily for exercise and they nibble on clover, dandelion leaves, and grass but it doesn't add up to a cup, and sometimes they dont even eat it.)
3. How much pellets? 1 ounce per pound of rabbit ( my 4 pounds holland lops get 1/2 cup) If you are feeding veggies as part of the day-to-day diet than you need to decrease the amount of pellets.
NEVER feed veggies to a rabbit under 6 months of age it can cause problems with their digestive tract and give them bloat.
I feed my herd 1/2 cup of top quality pellets, unlimited amount of hay, fresh water with added minerals and a tablespoon of apple cider vinergar (Per gallon of water) They get treats like cheerios or mint condition(my rabbits love mint)