What kind of filter? I rinse off my HOB
pads when I notice a reduction in water flow - about once a month or so. I replace the pads when they start to fall apart.
I really don't notice any change in my cannister filters, but I rinse off the pads and the cannister itself once every six months. I replace the fine pad and about 1/3 of the porous media once a year.
Of course, this is all done with tank water to prevent bacteria die-off. Ands I try not to do too much to the filter at any one time.