What a clogged filter actually does
A filter does not simply get dirty and stop cleaning. It gets dirty and starts choking. The blower is trying to pull a fixed volume of air through a mat that is closing up, so it pulls harder, and the airflow across the coil drops.
In summer, a coil starved of warm air gets cold enough to freeze solid, and a frozen coil sends liquid refrigerant back at a compressor built only for gas. In winter, the same restriction traps heat in the furnace and cycles it on its high-limit switch. Either way the part that dies is not the filter. It is the blower motor, or the compressor.
The ratio that makes it worth a calendar entry
Ten minutes, four times a year. The failure on the other side of it typically runs $200 to $600, and that is the ordinary version, where a technician replaces a blower motor. The version where a starved coil takes the compressor with it is a different page of the invoice altogether.
Nobody skips this because they are lazy. They skip it because a filter and a compressor feel like unrelated events. They are the same event, separated by about a year.
Pets change the interval
Hair and dander load a filter far faster than dust alone, and a house with shedding pets can close one up in weeks. Tell Homeward there are pets and the schedule moves the filter to monthly instead of quarterly.
It is the one task where the standard interval is genuinely wrong for a large share of houses, which is also why a generic seasonal checklist gets it wrong for them.
A note on thickness
Thicker media filters — the four and five-inch ones that sit in their own cabinet — hold far more before they restrict anything, and they legitimately run longer than a one-inch filter does. Homeward schedules the quarterly default because that is the filter most houses actually have.
If yours is a deep media cabinet, follow the manufacturer's interval and look at it on the quarterly reminder anyway. Looking costs nothing.
How to know without guessing
Hold it up to a light. If you cannot see light through it, it stopped being a filter some time ago.
Write the date on the cardboard edge in marker when you fit a new one, which turns the next check into a two-second glance rather than a decision.
Typical US repair ranges, not a quote. Homeward is a schedule, not an inspection, and it cannot see your actual equipment. Anything involving gas, electrical or structural work belongs to a licensed trade.