This depends greatly upon how well it is maintained but in general the average life of a furnace is 15 years. But I can’t stress enough that the better maintained your furnace is, by having seasonal check-ups performed every fall, the longer you can expect it to last.
My rule of thumb is when your system is around 10 years old and repair costs are getting high you should start looking into replacement. Another major tell is if your heat exchanger, the area that harnesses your fire in the furnace, shows rust and deteriation. In this instance you should definitely consider replacing your furnace. Heat exchangers that crack will leak carbon monoxide, a by-product of gas being burned off. This colorless, odorless gas will enter your home and is deadly to you and your family.