Dogs should get a bath at least once a month to every 3 months depending on how dirty the get and their coat type.
Dogs that are more active may need a bath more often than dogs that are less active.
If your dog gets often they should have a bath to clean them up regardless of how long ago it has been since their last bath.
Less active dogs only need a bath every few months unless the dog gets dirty and then the dog will need a bath to clean them up.
Dogs that are long haired may need more frequent baths, and may need a bath once a month, to prevent matting and prevent dirt buildup.
Dogs with short haired coats can often go longer between needing a bath.
And dogs who spend a lot of time outdoors and or play in dirt and mud will of course need baths more often than indoor dogs that don't get as dirty.
And some dogs with skin conditions or allergies may also require more frequent and medicated baths.
Also you should bathe your dog as needed, if they get dirty or get into something smelly or messy.
Bathing your dog too much can result in dandruff and scale and cause the dogs skin to dry out and cause the dog to become itchy after bathing.
After you bathe your dog you should also dry your dog after the bath to prevent matting, skin infection and the wet dog smell.
You can use towels to remove excess moisture after bathing your dog and use a hair dryer on low heat if your dog doesn't mind the hair dryer.