A pet rabbit's main diet should consist of grassy hay, but
Rabbit .org suggests the following guidelines for feeding apples or other fruits to your pet bunny:
"Fruits can also be fed in small amounts. In the wild these would be special high calorie foods obtained only at certain times of the year. Fruits make great training treats! You also might choose to hand-feed the fruit portion of the diet as part of developing a close bond with your bunny and also to make sure he has an appetite every day. It is a great way to see if your bunny is feeling good when you observe if he takes his fruit treat every morning! If he doesn’t want to eat his treat, it is time to call your veterinarian. Remember that dried fruits are about 3 times as concentrated as the fresh variety so feed less of those. Rabbits, like many animals naturally gravitate towards high calorie foods such as those high in sugar or starch. This is a protective device from the wild days when they could never be sure when or if they would get the next meal. When a plant would produce fruit, it is for a limited time and all the animals in the area would want to gobble these gems up quickly! This means that rabbits cannot limit themselves when given sugary or starchy foods if left to their own devices! Overfeeding fruits can result in a weight gain or GI upset so it is up to you to feed these foods in limited amounts. An approximate amount of fruit to feed your rabbit is a teaspoon per 2 lbs of body weight daily in one feeding or divided into multiple feedings.