Although there are no strict recommendations on how often you should bathe your dog, bathing frequency will depend on whether or not your dog needs a bath, and on such variables as the breed of dog, the kind of shampoo you are using, and how sensitive you are to the smells associated with dogs.
Most dogs usually never really require a bath. Baths can be given as often as is necessary (i.e. if your dog gets dirty or has an offensive body odor), although regular grooming, by way of combing and brushing, is often all that is required. Two or three baths a month is not considered excessive. However, if bathing is required more often than this, a special medicated shampoo should be used to prevent the hair coat and skin from drying out.
Human skin differs significantly from a dog's skin. A dog's skin is much thinner, has no sweat glands, has a different pH compared, and is generally much more sensitive than human skin. Shampoos intended for human use, even "baby" or "pH balanced" brands, should never be used on dogs. Heavily perfumed shampoos should also be avoided, since they may cause an allergic reaction.
The frequency of bathing rarely causes any significant problems in dogs, provided that the appropriate shampoo is used. In fact, the major cause of problems tends to be due to improper rinsing afterwards. A shampoo residue can cause skin irritations if left on sensitive dog skin, especially around the ultra-sensitive scrotum or vulva. If there is any doubt as to whether the shampoo is entirely rinsed, rinse one more time.