How often you should bathe your dog really depends on the type of dog you have and your preference. We have a Schnauzer in our family. For the most part, his head and back often are groomed very short by a professional groomer. Thus, he doesn't pick up much from the environment and stays fairly clean. However, he does have a long beard and skirt that seems to attract every leaf, stick and dirt particle from his big back yard.
Some of the things to consider when determining if it’s the right time to bathe your dog include: the dog’s hair/ coat type, hair length, where he spends most of his time (indoors or outdoors), shedding cycle, allergens in the environment and any skin issues that you are aware of.
Some dogs need bathed only a few times a year while others need weekly bathing. Brushing your dog regularly will help maintain the coat, avoid mats in the fur and keep your dog clean. I believe bathing your dog every month is reasonable. Some dogs will need more frequent baths or need to be fully groomed. A good rule of thumb is to bathe your pet when his coat gets dirty or begins to smell "like a dog".
You can consult with your local grooming salon or veterinarian for further advice.