I have purchased the FILTRETE1500 filters for years and replace the ones that look dirty after three months. For the ones (upstairs) that aren't as dirty, we go 4-6 months. The filters have made the air manageable for my children that had allergies growing up. I would recommend these filters to anyone that is looking to improve their home's air quality. One other benefit..... the heat/A/C contractor that works on our home's system said that these filters are extending the life of the system by eliminating dust/dirt from entering the ductwork and A/C unit housing.