I clean mine when I clean my powerheads. That is, when I remember to clean my powerheads
I guess this is about every 3-6 months or so. It always boggles my mind how much stuff grows in there. Its a haven for bubble algae and feather dusters (no coralline though
). Nothing grows on the sponge except for the dreaded "detritus". I really wish they'd come up with a better system than that sponge as I think its the source of my nitrate issues, or at least part of it.