Replacing a furnace can take 4 to 8 hours for a simple swap to 1 to 2 days for more complex installations involving significant modifications or new ductwork. The exact duration depends on the complexity of the installation, the type of furnace, and any necessary adjustments to the home's heating infrastructure. If the replacement requires extensive updates, such as new ductwork, structural changes, or updates to electrical or gas supply lines, the process could extend beyond a couple of days. These estimates can vary based on specific home conditions and the efficiency of the installation team, so it's best to consult with your HVAC contractor for a more accurate timeline tailored to your situation.