Hi, Bob. This is a little like asking how often you should put gas in your car. If you are driving a lot, you might have to fill up more than once. If you are average driving, you might only need to fill up a couple of times a week. And if the car is sitting idle, you don't have to at all. The next part of the answer depends on your chimney cleaner. A reputable guy will ask you how much you use it and give you a reasonable answer based on your usage. Another guy will say once a year, no matter what.
That's about as reasonable an answer as I can give you. But remember that there are things like dead animals and nests that can also cause you problems. If you check these out for yourself at the start of the season, you can save yourself a lot of problems also.