Well, this depends on the type of your dog and its lifestyle. Generally speaking, cleaning your dog's ear once a month as a part of their grooming routine, should do the job. Yet, for example, dogs that swim often, or certain breeds with a higher risk of ear infections, should have their ears cleaned more frequently (please consult your vet in such cases, since it is extremely important to avoid overcleaning, which could lead to dryness, irritation and other undesired side effects).