This one will vary a lot, I think, depending on your tank parameters. I was using just Oceanic salt for a while, and with the 550 or so CA reading, I was having to clean my Maxi-Jets about every 2 months or less. Corraline was everywhere: I had to clean the tank sides every few days.
After a recent calcium precipitation episode (another story), I had a layer on everything, everywhere, and cleaned all of my pumps.
My Pan world, Iwaki and Blue Line pumps have never indicated they needed to be cleaned (i.e. making noise, etc), but there was a fair amount of junk in them after the precipitation incident. I had run the the Blue Line and Iwaki (both 55 equivalents) about a year before I cleaned them, I think. Stuff does not seem to stick to that white type of plastic in these pumps as it does to that in the Maxi's, Mag's, etc.
Now with more normal Ca, I seem to be able to go 3-4 months with my power heads.
-Mike