As often as you need to. My old dogs rarely get a bath unless they need it. My dogs that are swimming get bathed much more regularly to keep them from smelling like lake water and my show dogs got bathed and trimmed at least once a week when they were showing. If you keep them combed out, that helps keep them clean and distributes the natural oils in their skin. A dryer also helps loosen dirt and flaky skin as well as blow out the dead hair.
So use your own judgement.