I change them when they get clogged. You can recycle them too. All you have to do is hold them at the edge with scissors and heat them to burn the crap and resin off as much as possible. Then when it has cooled, hold it flat on a table and scrape it all off with a knife. You should have a pretty much good as new screen at the end, and it only takes like 2 minutes... Handy for me, coz I always run out...