I think I went a year before the first change, but only about 3 months the 2nd time because I was cutting a lot of smoky ply and not checking the prefilters. I started to cut some acrylic and the odor came right through so I finally checked the filters and found the prefilters were gross and the charcoal was coated with resins. The most recent change was about 7.5 months, used maybe 85-90 percent of the time for engraving and the rest for cutting acrylic and ply. I didn't really need to change it just then, but knew it was a good time and would be a nuisance to do later when I had a bunch of ply to cut.