To be honest, I only fully bath my hounds when I'm aware they are smelling bad, or of course, if they have rolled in something dire. And I used to bath my bitches when they finished their seasons, before they went back in with the 'pack'. If they were moulting badly, I'd bath them too to help the dead hair come out faster. When I was showing them they obviously had baths more often, but my dogs are basically clean (and going out in the rain helps!!) so the answer to your question is not that often, and it's a matter of personal choice really, within reason.
And constant bathing does dry out the coat, although if you use a good dog shampoo, that helps. Some breeds do need regular bathing however - eg. Afghans.