I don't have allergies or breathing issues, but I do have shedding cats.  After less than a week, the prefilter is visibly dirty (meaning it's working) and the filter is a little dirty.  I keep it on the lowest setting all the time so it's not too loud while sleeping. I haven't played with all the settings yet, but it has a timer and a LED display so you clearly know which setting you're on.  The prefilter should be changed every 3 months and the filter once a year, which is nice.  Looking around at different sites, both filters are pretty reasonably priced.