I take my girls a bath every week. They get pretty stinky by the last day and if my bf says "phew" when they come into the room then it's time.
(Actually they're not THAT bad, they just get the doggy smell). I think it really depends on their coat (some of them get oily fast) and their skin. If they have dry itchy skin it's better to wait longer between baths and use an oatmeal shampoo. I hope this helps.