The answer to 'how often to replace furnace' is that most furnaces should be replaced every 15-20 years, depending on the make, model, and usage. Proper maintenance and care can extend the lifespan, but eventually all furnaces will need to be replaced as they become less efficient and more prone to breakdowns over time.
It's important to have your furnace inspected and serviced annually by a qualified HVAC technician to ensure it is running safely and efficiently. They can identify any issues and let you know if replacement is recommended soon. Factors like the age of the unit, energy efficiency, and repair costs should all be considered when deciding if it's time for a new furnace.