I've always heard for a puppy every two weeks. But I now bath mine once a week. If there are any cling-ons then I do a butt wash only, as needed. I was using oatmeal also and I can't say I really noticed much of a difference. Uni is an itchy baby. I recently started taking out all grains in her diet, and adding virgin coconut oil in her food and on her body topically and has worked really well so far.
Biogroom is a good shampoo I use. The one I'm using now is really good but I forget the name. I just started using it and I'm not at home right now, but I bought it from mimimomo, who is a member here. She has good stuff and good prices. Also I love the Madan brushes she sells. check her out at toplinepet.com or PM her.