As a rule of thumb, kittens can be bathed from four weeks of age, National Cat Groomers Institute has a terrific demonstration on Kitten Bathing.
Shorthaired cats not often than three times per year during shedding season unless the cat gets filthy. Longhaired cats’ benefit from regular baths every six weeks and show felines a week before a show.
In longhair breed cats such as Norwegian Forest, the undercoat can be thick. The coat collects debris from around the house/garden and tends to form tangles. Neglected snags can turn into mats so pet owners need to lend a hand every four to six weeks. In cases with pelts, a professional cat groomer will need to be enlisted.
Cats with rippled coats like Cornish Rex and breeds with long curls do not shed heavily. Forceful grooming can impair the fur appearance so bathing instead of brushing is recommended for similar type cats.
If a cat is extremely aggressive, a bath and blow dry should be performed more often by a cat groomer to avoid matting that might result in the cat needing to be shaved. This is also important for cats with thin skin or low body weight.
Bathing geriatric or ill health cats may be too stressful so consult your veterinarian’s recommendation.