That all depends on your pet.
Factors include: his hair coat, hair length, how often he gets dirty, where he lives (if he is indoor or outdoors most of the time), shedding cycle, and any underlying skin problem.
Some dogs need baths only a couple times year while others need weekly grooming. It is beneficial to brush your dog about twice a week. Bathing your dog every month or two isn't unreasonable, but some dogs will need more frequent cleanings.
A good rule of thumb is to bathe your pet only when his coat gets dirty or begins to smell "doggy."