I've heard that it's not a good idea to bath a dog often. What is considered often?
More than once every three to four months is probably too often. Shampoo can strip the natural oils from the dog's fur and skin.

If your dog gets really dirty and desperately needs a clean-up, then simply hose him off with plain water between baths. If he has an unpleasant odor, then spritz him with doggie deodorizer.
It depends on the individual dog, but a general rule of thumb is no more than every two weeks. Once a week if absolutely necessary, but not all dogs can tolerate such frequent bathing. Use an all natural, gentle, soothing oatmeal & aloe shampoo that won't strip all the oils from your dog's coat.

I have to bathe my border collie, who is a working therapy dog, every time we do a visit to a hospital, school, or nursing home. Because of the bath requirement, they are required to visit twice a month, but more visits are up to how much bathing your dog can tolerate. Right now he is doing great at every two weeks, with great skin and a soft, shiny coat.