Depends on the breed. If it's a dog that doesn't require grooming as frequently (say a Labrador) then I suggest every 2 weeks. If it's a dog that does need grooming (say a shit-zu) then I suggest every 5-7 days.
It truly depends on how active your dog is, how dirty it gets and if it has skin, or fur issues. If you don't want your home to smell, wash your dog at least once a week; more if your dog tends to get dirty faster or has issues with body odor.