mheidershscheit,
I am not clear as to what type of a "filter" you are referring to here. Do you mean a mechanical filter which simply strains out solid debris and detritus or is it some sort of a bio-converter?
If it functions as a bio-converter, then I would assume that you would only wish to rinse it off gently with some of your clean pond water to dislodge some of the trapped debris, but you would not want to scour it completely or harshly because that would remove the bacteria cultures that may still be living on the surface. You want to maintain them and keep them alive to perform their ammonia / nitrite reduction duties. The less you disturb these items, the better.
If it is just a mechanical filter for straining out solids, then it really is dependant upon how much solid matter that they collect. Clean them out before they get gunked up and reduce your water flow or anytime you have spare moments to do a PM (preventative maintenance) on the filter system. For a mechanical filter system the cleaning schedule would be based upon each individual pond system and you would determine this schedule simply through trial and error and monitoring / observation experiences. There is no set rule because every pond would be different and the size of the filter would be unique. You simply clean it when it is dirty and learn how long it takes for it to become dirty again to know when to repeat the cleaning cycle.
I think you will be able to identify when the water flow is diminishing just by observing your system. You'll know when it doesn't look appropriate to you and then you can check and clean your filters. Just keep it simple and you will be fine.
Gordy