The simple answer to this question of how often to bathe a dog is: how often do you want to bathe your dog? Unless a dog has skin problems or severe allergies, there is no specific need to bathe the dog except to make him a more enjoyable companion.
Just like people, some dogs get stinky faster than others.
General Dog Bathing Guidelines
Dr. Sherry Weaver recommends you bathe a dog with normal skin once a month with dog shampoo or human baby shampoo. If you want to bathe more often than once a month, use a soap-free or moisturizing shampoo to prevent the skin from becoming dry. Do not bathe your dog more than once a week, unless recommended by your vet.