The filter only needs a "clean" when the flowrate drops to an unacceptable level. Experience will tell you when, but you might like to start at 50%
You MUST ALWAYS clean any filter media in old tank water, as doing it "under the tap" will kill all the "good" bacteria you have been building up during the "cycle" - its the chlorine that they add to tap water specifically to kill bacteria that does it, INSTANTLY...
So best thing is to combine it with a water change, syphon out some water - clean the media whilst youre at it !