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A good rule of thumb is once every 6 months in a vinegar solution but if they become clogged then obviously clean them.. yours still looks pretty good though so I wouldn't worry about it
I dont clean mine for say... i simply take the cover off and tap the propellers a few times. My power heads are incrusted with coraline. I do this ever pwc.
I check to make sure there's nothing that's stuck in there but I soak one at a time in vinegar and water about every 3 months. I tend to overdo things so I'm probably not a big help! Just check for debris every once in a while when you clean your tank.
I think in the year I have had my tank, I did clean them once bc I had and algae bloom. My sea urchin and snails love to clean them for me so I haven't cleaned them in awhile.