Unlike humans, who generally have fairly similar bathing schedules, how often your dogs needs washing varies much more.
As a general rule once a month works for most dogs, but their lifestyle can increase or decrease the frequency they might need. Does your dog often have walks through forest, parks or other dirty and muddy areas? They will definitely need washing more. Conversely, if your dog spends most of its time walking on concrete it may needs less regular baths.
The dog's place in the family has changed over recent years, with many dogs living inside, and being allowed on the couch (and even the bed!) so it's important they are clean and smell fresh. Clean dogs get more cuddles!