With a few exceptions, never is best. Brushing is much healthier.
How often should I bathe my pets? I’ve heard everything from once a week to never.
In almost all pets the right answer is never. No kidding. Here’s why. Unlike humans, pets don’t have sweat glands. Without them there is really no hygienic need for frequent shampooing. But pets, especially dogs, do start to swell funky without grooming. This is because, like all of us, they normally accumulate skin flakes and dead hair on their skin. But each time you use shampoo you wash away an important layer of protective oils that serve as a barrier to skin disease. So the best method is to use a brush on a short-coated pet or a metal comb on a long haired pet. This stimulates their scalp and pulls loose the dead hair and skin flakes. They smell better, they look better, and they feel real good too. Best of all, their skin stays healthy. If they get muddy, you can rinse them with water. Water by itself is fine as often as you want.
By the way, cats usually bathe themselves with their tongues. If you insist on bathing your cat yourself, be sure to rinse the cat hair off your tongue when you’re finished. Otherwise you’ll get a hairball. (just kidding)