I have cocker spaniels, a breed of dog known for chronic ear problems.
I make my own ear cleaner that is safe to use on both dogs and cats (with no wounds in the ear and so long as the ear drum is intact). It is a mixture of alcohol, vinegar, boric acid, and betadine. It is relatively inexpensive to mix but works just as well, if not better, than the solutions you can buy from the vet. Saves us a lot of money in the long run than buying those very small, but very expensive bottles of ear cleaner from the vet.
There are some of my animals that I can't use this solution on, so instead I make another homemade mixture of water/vinegar/witch hazel. You simply squirt it in their ear, massage their ear/ear canal, let them shake their head, then wipe out debris with a cotton ball.
I don't clean the cat's ears unless I see some dirt in them. (I give physical exam every time I clip their nails about once every week, or once every two weeks.) The cockers ears I clean religiously, dirty or not, every week, if not more often. But for healthy, clean ears, I likely wouldn't feel the need to clean them more than once or twice a month.