If there is sucha thing as an 'average' aquarium and fish load, I'd say every 3 weeks is a good idea. We are alternating on the 2 canisters we have, and clean one each week at the time we do the water change.
I do 1/2 of mine one week, then the other 1/2 the next week. IMHO it makes cleaning the canister quicker and easier than being left to clean after a month or so of use.
We have Discus and need to make sure we have exceptional water........
And why not provide your fish the best water you possibly can?
Come on --- what's the going rate on filter floss?? And - a little extra time on the maintanence?
I suggest for those that go 2 months (and even more...) without at least 'checking' to see how much 'build-up' is in there, to try after 1 month. You are also losing suction and effectiveness having a clogged filter.