It depends on whether it has a short coat, long coat, or a double coat. Also, it varies according to the season, the area you live in, and the breed of dog. Some long-haired dogs should be groomed every four to right weeks, and “summer cuts” are typical in hot climates. On the opposite end of the spectrum, some short-haired breeds can go a year without going to the groomer. However, they can also be bathed weekly with good results.
Dogs with double coats are more complicated, since their undercoat usually has protective oils. Extra attentive grooming is required when the animal “blows” or sheds its undercoat. Otherwise, it varies by breed, but usually once every few months.