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Only as often as needed, so it depends on the piggy. My short coated girls maybe one, twice a year tops. More than likely once in summer. My long coated girls get regularly bum baths, like once a week.
Some recommend a monthly bath, but at @Flutterby says, it depends on the piggy. I bath my long-hairs pretty frequently, and clean up the grease glands on those short-hairs that need it. I also give everybody a shampoo in antiparasitic (e.g. Lice n Easy) or antifungal shampoo (e.g. Nizoral) every few months.