MOM says:
"Inspect every 6000km - 4000mi - or 6 months"
"Replace every 18000km - 11000mi - 18 months"
I suggest that if you are going to go the trouble of inspecting it that you (at the very least) tap out all the accumulated goodies. Better yet blow it out, from the inside to out (airflow wise) gently with compressed air. go gently pleat by pleat. If you blow a hole in the paper you will have to replace it a little early,
Steve
(MOM aka Motorcyle Owner's Manual, although this time I found my factory service manual first.)