When does a owner need to replace one of these filter elements? Is it replace on time or hours of use or some other indicator?

Usage can vary greatly, so there's no hard rule for when a HEPA filter needs to be replaced. We would recommend visually inspecting your filter twice per year. The filter can be cleaned by fanning the ridges and tapping the filter on a surface. We do not recommend using compressed air to clean the filter, which can compromise its material. With visual inspection, if you seem damage, holes or excess dust which cannot be cleaned, then replacement is needed.