I have an old sears shop vac that I hook up to my SCMS and use for shop cleanup. I can't remember when I changed the HEPA filter. Every time I empty the vac I take it outside and throw it up in the air and let it bounce on my drive several times and then use my Festool vac to clean the filter. The outside is filthy, the inside is pristine. When should I change it, frankly I can't remember when I bought it but I am sure it is over 5 years old.