My dog is 19 months and has had 2 baths. The vet says her coat is wonderful and I don't want to push that. She really doesn't do anything that gets her dirty and she doesn't smell. She is a very low maintenance GSD, I only brush her every now and then, she just doesn't need it. If you think your dog needs it, just make sure the shampoo used is gentle enough to be used all the time.
Try just using conditioner only. It will still clean and freshen but it's a lot more gentler and it'll leave a lot of the oils intact so the coat won't overproduce oil. It also won't mess with the pH of the skin.
I have a dog that loves to dig and get covered in dirt/mud. Sometimes I just hose him down in the tub or do a light wash. It's about 1 a month or every three weeks. Between baths I use doggy wipes (I use pethead) to wipe off muddy paws and legs (rainy Washington so tons of mud).
I consider once a month too much - others will tell you it is fine and part of their grooming routine. If you have been giving Saleen baths on a monthly basis, and she is fine with a good coat, all is well.
If her coat seems dry and brittle, the frequent bathing could be drying it out.
As long as you are using a mild, gentle shampoo that doesn't strip the oils out of the coat, you can bathe as often as you want. Personally, I only bathe mine when she needs it (every couple months or so) but I have clients who bring their GSDs to me once a month. I think every 3-4 weeks is about the maximum I would recommend, unless the dog gets very dirty or there is a skin condition needing specialized shampoo.
I use professional grooming shampoos at my salon, but Earthbath is a nice, mild shampoo that is widely available to consumers.