Once a month or when the flow from the filter slows down. It depends on what is in the tank as well. If you have big fish that are messy eaters the filter will get dirty faster than if you have a few smaller fish. My canister filters go from six weeks to two months between complete teardowns for cleaning. Magnum canister filters have a gross partical floss filter pad on the outside that I rinse throughly and then I change the charcoal in the chemical section. It takes about ten minutes to finish. With an outside filter like yours you may be able to extend the service time by rinsing the cartrage.

Just rinse itwith some removed aquarium water from a weekly partial water change. I have the same one running on mine as I did over a year ago. Just replace the carbon if you use it when needed.