I clean mine whenever the flow starts to slow noticeably, every 6-8 weeks and then I just rinse the sponges and check the impeller for gunk buildup. The interval will depend on how stocked you are and the make of filter. Also, if you have carbon in there, it will need regular replacement (if you follow the manufacturers instructions, I don't use carbon at all, but keep some ready in case I need to remove a medication).
Jon