My pond is 1500 gallons and is overstocked with about 30 fish. Koi, goldfish and shubunkin. Six of the koi are quite large at about 20".
I usually clean when the outlet pressure reduces.
One of my setups is a submerged pump feeding a pressure filter with a UV exiting over my waterfall. I remove this setup for the winter because I don't want the water from the falls being pumped on top of a frozen pond.
In the spring it gets reinstalled. There is a period of a month or so where this setup gets clogged quickly until everything gets balanced and caught up. During this time I clean the filter pads and the pump inlet holes when I notice the pressure reducing. Usually once a week. Eventually, with the filter running and all the plants starting to establish, the time between cleanings becomes once every 2-3 weeks or if I see the pressure reduce.
My second filter runs 24-7, 365. It is a bucket with a pump on the inside bottom covered with lava rocks. The outlet of the pump is a piece of 3/4" PVC that protrudes out of the bucket and stops just below the water surface creating my fountain. When I see the height of the fountain reduce, I pull the bucket and rinse everything out.
I also have aeration that runs 24-7, 365.