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.