My current recommendation is once daily for dogs and cats. The reason: bacteria are able to build a complete layer of protein along the gum line in a 24 hour period. Disrupting this process markedly reduces the risk of periodontal disease.
Full disclosure: my pal Buster gets his teeth brushed five or six times each week. I don’t follow my own advice religiously, but I do come close. Buster is now six years old, without a hint of gum disease.
Photo: if only they did it themselves!