There is no specific time to bathe your dog, unless it’s the obvious, like when they have been out rolling around in dirt and grass.
Some need baths more often due to skin problems
Others need it more frequently because they get stinky faster. Like humans they get smelly to.
Other than the obvious, once a month with a pet shampoo and conditioner is best to keep skin and coat healthy.
A bonus is many dogs enjoy the message and connection with their owners during bathing.