On average, most well-maintained furnaces will last somewhere between 15 and 20 years. Here are some factors that will affect the how long your furnace will last:
The type of fuel - In general, oil-fueled furnaces tend to last the longest.
Proper size - If your furnace is too large for your home, it’s going to turn on and off a lot more, which puts extra wear and tear on its components. Likewise, if it’s too small, it might have to run nearly all the time, shortening its lifespan.
Poor ductwork - If your ductwork isn’t installed correctly, is the wrong size or has too many leaks, this will impact the workload of your furnace which has a direct impact on how long it lasts.
Wrong thermostat settings - You shouldn’t overwork your HVAC system twith inappropriate temperature settings. A temperature set between 60°F-80°F is ideal.
The most important factor, however, is if the furnace has been properly maintained. It’s vital to take care of your system to ensure it lasts its full lifetime, and maybe even beyond.