I would say once a week is good - please let me in on your secret - how do you do it??
I haven't attempted to clean their teeth as yet - i know you can buy special cat toothpaste just for the job. Apparently you have to get them used to just a toothbrush first before you even attempt to brush the teeth with paste, is that right?
However, my vet suggested that unless you've got a very patient or tolerant cat, then the best you can do is feed the Oral care biscuits which are larger in size and make your cat bite down on the biscuit, rather than swallow it whole or just crunch it in the back teeth.
She didn't mention that she would clean their teeth for me?
I may ask about it next time i'm at the vets, my cats are due their booster injections next month anyway, so i'll let you know what my vet suggests with regard to 'professional cleaning'.