I have not cleaned mine in over a year, but I have another hob filter that actually does the majority of the filtering. I also use a prefilter over the intake, which cuts back on the gunk that enters the canister and hoses. That may not be practical if it is your primary filter however.
You should back flush it when you see the flow diminishing. Place it near a sink and remove top. then remove filter pad or batting if you use that. Position the intake nozzle so it points into the sink. Now take aquarium water and pour it in the top of the filter. The water will leave the canister and pour into the sink, taking the crud with it. Do this a couple times.
Replace filter pad or batting and reassemble. Hook it up and you are ready to go.