I was asked a few questions about taking care of a rabbit's litter box such as how often do you clean it out, do you dump the whole thing each time?
It depends on the litter. If you are using something like wood shavings, shredded paper, or hay in your rabbit's litter box, it should probably be dumped every one or two days since these beddings aren't very absorbant. Hay should definitely be dumped every day.
I use recycled paper pulp bedding which is very absorbent. Every day I spot clean the litter box by cleaning out all droppings and wet or soiled parts, and then adding some fresh bedding. Once a week I completely dump out all the contents of the litter box and rinse the litter box out with hot water and sometimes a very small amount of vinegar to remove the mineral deposits from the rabbit's urine.