For most breeds, healthy individuals very seldom need to be cleaned. Rabbits are very tidy creatures, and will do the vast majority of their own grooming themselves. They will lick their fur, and use their paws to get at those areas that are a little tricky to reach. It’s likely that their friend (or friends) will give them a hand too - you will often see your rabbits grooming each other. Not only does this help them bond, but it also makes it easier to keep everyone nice, clean and parasite-free.
Most breeds will only need cleaning when they're really filthy
If your rabbits are outside often, or have very long hair, then you may need to groom and clean (but not bath - baths are quite dangerous for rabbits) them once in a while, however, this will depend strongly on which breed you have, and how dirty they will get where you are keeping them. For example, rabbits of the Angora breed need grooming every day!
One of the few things that you need to do is regularly check your rabbit’s behind for faecal buildup. If this does occur it makes your pet really susceptible to a horrible condition caused by flies - flystrike. It’s often fatal, so every precaution should be taken against it. Keep your rabbits, and their hutch, clean and as free of faeces as you can manage. Spot-cleaning once or twice a day when you feed your rabbits is a good way of reducing the risks.