On Wednesday, February 6, 2019 at 3:16:22 PM UTC-5, Scott Lurndal wrote:
A dirty filter:

1) Reduces the airflow available to the heat exchanger. This could result
in the high-limit switch opening, which will increase the cycle time
(and blow colder air util the high-limit switch closes after the heat
exchanger cools sufficiently, rinse and repeat).

2) Causes the blower to work harder, which consumes additional power and
prematurely ages the blower.

A dirty filter does a couple of things.

It reduces air flow, perhaps very slightly to a lot.

It increases the efficiency of filtration. Dirty filters catch more dirt and work better, up to the point where reduced air flow becomes a problem.

If you reduce airflow enough, you might have coil freezeups or other problems. Residential systems tend to need a steady airflow across the coil, balanced to the temperature and the amount of charge. The old ones aren't intelligent enough to adjust, I dunno about more recent ones.

If your filter is dirty enough, theoretically it might rupture and spill unfiltered air into the equipment. The purpose of a filter is mainly to protect the equipment, not the humans. I haven't seen this happen in a residential system but I have in a commercial one. So I don't know if that's really a problem in a house.

I change my filter when I hear the sound in the return increase. That's probably about 3 months or so, I don't keep track. I just cleaned my refrigerator coils last night, they were caked with dust. (I have that stupid double A coil setup, where you can only reach the outer two rows.)