Monthly. Rinse the foam inserts until they are clean. You can reuse them until they begin to fall apart, then replace them. Change carbon monthly. Never change the bio-rings, and rinse only in water from the same aquarium. Flush the hoses, and clean the plastic tubing.
Check the outlow daily to make sure the current is strong. If it's diminshed, something is clogging the filter and needs to be addressed. You can lessen the likelihood of this by using a prefilter to keep large particles out of the cannister.