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If you want to play with toys, not guess, and always know when the
filter needs changing put a filter-check device on your furnace (like
is used on large engine air-filtes). It is just a differential
pressure guage - can be as simple as a manometer tube connected across
the filter - from the air return duct to the return plenum of the
furnace. A plugged filter will have a higher differential pressure
than a clean one.

That is what we used at work on the large building air filters. They had a
motor on them so we could roll them up some when the pressure differential
reached a certain level. They were on a large roll at the bottom and the
takeup roll at the top.

There was a pickup tube on each side of the filter.
Some were gauges with alarm switches,and some were simple fluid indicators.