Living with a House Rabbit by L Dykes & H Flacks (I give this book so much praise) recommend you dont give more than one piece of fruit per day, aiming for one tablespoon per 2kg of your rabbit's body weight.
Personally, i wouldn't give my rabbits this much. I have mini lops and think one whole apple each, for example, would send their digestive system in to chaos!!! I certainly wouldn't exceed this given the high sugar and acid content in fruit which could cause dental problems.
For example, I give Dexter and Lily half a banana each in the morning and sometimes (if they're good) a couple of cubes of apple, along with their pellets. They have the remainder of the banana with their evening greens. If we have them we give them one or two grapes each, they like strawberrys, peaches and pears.
I would say if their droppings look like this they are having too much fruit, cut down a little and ensure plenty of hay is available and this should aid the digestive system get back to normal.
Hope this helps,