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 regular tune-ups can extend the lifespan of a furnace, but eventually all furnaces will need to be replaced as components wear out and efficiency declines.
When a furnace reaches 15-20 years old, it's a good idea to have a professional HVAC technician inspect it and provide a recommendation on whether replacement is needed. Factors like the furnace's condition, energy efficiency, and repair costs will all play a role in determining if it's time for a new furnace.