Check out this product, Life for Teeth. You can also do a search and find other threads where people tell how they like it. It may eliminate vet teeth cleanings all together. They recommend once a year for cleanings, but it really depends on what type of diet yours is on. If they only eat dry food and are given bully stix to chew on they can go a lot longer in between cleanings.
http://www.tobysnaturalpets.com/petz.htm