As you say, depends on the bio-load, tank size, stocking density, foods being fed and water change frequency.
When I used them, I always had a sponge filter running and would alternate cleanings. I used to break down my canister weekly (most would not). Its mainly because I fed 10-12 times a day and waste builds FAST. I used a prefilter in the intakes and still, the canister would get dirty fast. In the 5 or so months I used my canisters, I never replaced a single thing on them, only rinsed/squeezed the sponges/media in tank water. My canisters were mainly course sponges and pourus rings.
HTH
Eddie