The expected life span of a furnace is totally depends on how well it is maintained; whether or not it was installed properly and how often it is used. Most of the times, you can expect a properly installed and well maintained furnace to last around 15-20 years. But, if your furnace is not maintained properly, its life expectancy can decrease by a several years.
http://www.romaniukheatingac.com/what-is-a-furnaces-life-expectancy-and-signs-of-a-dying-furnace/
