When we first got Jaxon, I was using a baby shampoo and it really dried out his skin. I bought Chris Christensen shampoo and conditioner for him - it seemed expensive at the time but it made a HUGE difference. We bath him every week and still have over half a bottle of both the shampoo and conditioner.
I can't imagine going 2 weeks or more for a bath. He starts looking bad by day 5 or 6 and if he's been outside a lot or around other dogs, he can start really smelling like a dog!!
I also have the Chris Christensen detangler/leave in conditioner which is great for day to day brushing and also after a bath.
http://www.chrissystems.com/