just when they are dirty , maybe once a month - 2 months , unless they are bad and get into the mud or something. Mine are indoor dogs so not like they live in the dirt. Over bathing can cause dry skin and loss of natural oils resulting in irritation, flaky skin, dull coats.
In the summer I give Mel an oatmeal bath like once a week, in the winter I do it at least once a month because he gets really dry skin, even with the fish oil he gets. It makes him really soft and his skin is not as dry.