Usually let the Vet decide
My previous dog at her teeth cleaned at about 5&8. If your dog is prone to infections, ask for a wide spectrum antibiotic to be started before the cleaning (e.g. doxycycline).
Great Teeth cleaning product
Cleaning the dog's teeth is an excellent thing. It helped an older adopted dog of mine to clean his teeth. He was able to eat dry foods again.
There is a great product out there that I used. It is "Dental wipes". It is a good thing for in-between cleanings. The brush doesn't always work out too well with some dogs. I find the wipes easier to manipulate and use instead. However, it may take some time to get the dog adjusted to doing the cleaning.
My suggestion is to follow the instructions on the label. They may sound strange but it is understandable once used. The label suggests holding the dog (if possible) and reaching from BEHIND into the dog's mouth with the wipe. Kind of like how you would do a drowning victim. Always from the behind as not to startle. I usually rub on the teeth I can get to and then later maybe the gums. It is amazing how much plaque can come off just by wiping.
You can find these wipes at a Pet store. They are over $5 but you get a good number of them. They remind me of the normal wet wipes. I hope they work for you if other methods don't. Good luck!!!