A modern heating and cooling system will function for long periods of time with virtually no maintenance at all. It can in fact, take several years for a system to deteriorate to the point that something breaks.
Since this equipment is almost always out of site it is easy to take it for granted. If the room temperature is acceptable, we tend to forget about it. This can be a dangerous situation.
Even though the equipment may be functioning it is likely working harder than it should, wasting energy and shortening its life expectancy.
The two items which most often affect the performance of the unit are filters and belts.
When it reaches its holding capacity the dirt will begin to unload, or pass through the filter and be caught by the coil. All of this restriction in the system will cause it to run longer to satisfy the thermostat.
When the coil plugs, the air can no longer pass through it fast enough and it will freeze up. Typically this would result in a service call to replace the filters and clean the unit. This is what most service agreements are based upon. If you change the filters before they start to unload, the equipment will function.
The question is, how much energy are you wasting through excessive run-time when the filters are not changed regularly?