On my wet dry i can see a small "flood" appear on the side of the acrylic. When i see that, it means that my filter pad is clogged and needs to be cleaned. I do it once every 2 weeks.
On my canisters, i clean em alternately every other week. Im diligent with the wet dry and canister on my ray tank because , well , there are rays in there.
On my other tanks, i just clean the canister once a month and the hob or ohf even less... LOL