The standard 1” air filter is factory stamped as a 30 day filter. Although thisfurnace air filter may seem a bit overboard the concept is very sound. Both Heating & A.C. units need to move air in order for them to work easily and efficiently. A plugged filter could result in costly repairs that can easily be avoided with some routine maintenance. Change your filters regularly!!!

More interesting information can be seen on the links below:

How to change your filter –

Home Inspector finds a home with no filter in it.  – Home inspector blog

The average person breathes in about 32,000 pints (18,185 liters) of air per day. Those 32,000 pints contain about 1,120,000,000 particles of dust –

We carry many of the  common furnace filters sizes on our website