It depends on the diet and the dog. Annie our old Newfie had her bottom jaw amputated due to cancer. We brushed her teeth once a week simply because she only had 2 bottom molars left. Ive not had to brush Bell or Hovis's teeth yet, with what they chew and eat their teeth are always clean and there is no sign of problems. I do however check my dogs mouths a couple of times a week, it helps them get used to it, if the vet needs to examine them or they need tablets and I wouldn't hesitate to use a toothbrush if required.
If I'd not been doing this with Annie, I would have missed the tiny black lump and lost Annie 7 years earlier.
Andy Bell and Hovis