Cats typically do fine washing themselves. If you find your cat is dirty you can try bathing him once a month using a special grooming spray. if your cat has fleas than you will want to give him a bath using a flea shampoo.
1 Additional Answer
Unless your cat has rolled in something especially stinky or messy, they should not have to be bathed. Cats have excellent grooming skills, and this alone should be enough to keep your cat clean.