Biggest problem is the ridiculously small size of the filters compared to the airflow pulled through them in a normal working day. We clean ours monthly and come the following month, some look as though they have never been cleaned ! As for the projectors fitted in the Home Ec dept ……..
Some of our projectors require cleaning more frequently but we do not have the manpower to do that, which brings me to my next comment. I would have liked to have asked the Epson trainer mentioned by CapEnd earlier, why projector manufacturers like Epson fit such small filters in the 1st place and then expect them to last a reasonable time without clogging.