When I go to buy air filters for my central heating and cooling unit, I like to buy them in bulk, and if I do not buy in bulk I like to buy at least 2 large packs.
The reason for this is to save myself headache, gas and time from having to keep coming back to the hardware store every single month.
I change the air filter of my central heating and cooling system method once a month even if it does not look or feel like it needs to be changed. This is a way to ensure that my central heating and cooling system method is working at the full power it was designed to. It also helps with keeping the air vents and the air duct wipe as well. I only get our air duct to my central heating and cooling system cleaned once a year, but occasionally I may even skip it if it seems that there is no problem and the local heating and cooling specialist reports no issues when they do their heat and air conditioning system tune up and check up every season. I think I am pretty much set in terms of air filters for my central heating and cooling system unit. I bought enough this last time to really last me about a year or maybe even more! I also should mention that I buy washable air filters too, which extends the use of them, but washable air filters are pretty cool I have to tell you!