If you change your furnace filter very often your system will last longer than average ac or furnaces. Please remember if your furnace filter is not clean it clogs the fins of evaporator coil and eventually makes the evaporator coil completely clogged. So please make sure you change your furnace filter before it get extremely dirty. If filter manufacturer recommend that filter should be changed in 60/90 days. please change it at least 15 days before dates. if you maintain these filter changing schedule strictly, your ac or furnace should approximately 15 years.