I have been trying to determine how long to let the filter go before changing it. With new 'stats that measure run time, I have an answer that some might find useful. What follows involves some anecdotal data.

MERV 8 Pleated Filter life Calculations.

Based on watching a filter get dirty to where the flow was noticeably restricted (too much dirt)

Household family of 4 without pets

Filter was a MERV 8 pleated filter.

Only one filter grille with a 32X16 filter

120 hours of use based on digital thermostat timer

Blower set to 800 cfm

Total cfm of air that passed through filter was--

120 Hrs X 60 Min/Hr X 800cuft/min = 5,760,000 cu ft of air filtered.

Filter area= 16 X 32=512 sq in.

Volume of filtered air/Sq In is 57600000/512 = 11,250 cu ft/sq in

Since the filter was too dirty, I'm going to use 10,000 cu ft/sq in as my ideal volume per sq in.

Now, the ideal filter is sized for 2 cfm/sq in of filter area.

Therefore, the expected life of a filter on a properly designed system would be--

10,000cu ft/sq in divided by 2 cfm/sq in= 5,000 minutes or 83 hours.

A more messy household would shorten this value. A retired couple could lengthen it.