I have two Trane air handlers for my forced air system which covers over 4000 sq ft. I typically replace these every 6 months or so. This is my second round of purchasing these filters and they do the trick. A little more expensive than some of the other options but you have to pay for quality sometimes and I am ok with that.
We have 2 cats, 2 dogs, and our oldest son has asthma. He got the diagnosis almost a year ago. Since then, we have been using these filters to better the air in our home. With all the animals, and living in Georgia where pollen rules the spring, we have noticed way less animal hair, dirt and pollen in our home. We change them every 30 days, and they definitely filter a lot of crap out of the air. For areas high in pollen, or dust if you are in the desert, or if you have animals, I highly recommend these filters. Minimal dusting of furniture.l, and you can't even tell we have animals! Plus, the best part, my son breaths better.