@nprofile1q... One of the upsides of HEPA is they're supposed to 'fail closed', ie. it never lets dirt through more it just eventually lets air through less. Which means what you're looking for is the monetary sweet spot on power usage to airflow ratio within adequate airflow for room air changes range. In my anecdotal experience, the airflow drop and the looking a bit past ucky coincide except under weird dust conditions. However, I also find the side benefits of things like carbon prefilters fall off faster. I've almost always, when I finally changed filters, felt I could beneficially have done so sooner.