I wash my dog once a month, but only because I have slight allergies. When I start to break out on my chin, I know it's time for a bath for the dogs.
6 times a year or less is preferable, but you want to do it once a month MAX, no more then that
EDIT: You NEVER want to wash a dog more then once a month. For one, it strips the natural oil off the skin and fur, and this is actually what helps keep your dog clean. I.E. your dog actually gets dirtier and smells more from over-bathing. That lack of natural oil can also cause extreme dandruff, irritated skin, and can break and ruin the coat, making it very brittle and unhealthy. It's just not that good to wash your dog that much.
· 1 decade ago