Properly maintained Hvac systems last 10-15 years on average.
The top two causes of premature system failure is improper installation and improper maintenance. Manufacturers recommend that your home comfort system be maintained no less than twice a year.
R-22 (freon) refrigerant is no longer in production due to environmental impact and has been replaced with 410a (puron) the new refrigerant. What this means for you as a customer is that although R-22 is still available there are ethical limits for its use, so the epa urges us to repair leaks before large amounts are added. http://www.epa.gov/ozone/title6/phaseout/22phaseout.html
Filters should be changed as soon as they change color (show gray) to optimize system longevity and performance. Use only pleated filters for best results. If you use m.e.r.v. 4 or lower your nose is the main filter in your home.
Seer rating is a seasonal average of the equipment's efficiency/ usage.
Systems that are not "matched" will not work, all new systems are rated by the ahri.