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, 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. I have known dogs that have never had a bath in their life and do not have an unpleasant smell, but my own dogs get bathed monthly.
General Dog Bathing Guidelines
I recommend 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.
How about you? How often do you bathe your pooch?