I think it varies, depending on how much you walk them as the concrete "files" them. I thought 6-8 weeks was sufficient. When I take Joey to Petsmart, I ask the groomers if they think he needs it, and sometimes they tell me no and that I should come back in a couple of weeks.
If you are home clipping, I don't think doing it more often would be bad, as long as you are just taking off tiny bits. I'm so afraid of hitting the quick.
