To maintain a properly functioning furnace, cleaning or replacing the furnace filter every one to three months is recommended. The frequency depends on factors like the type of filter used, the air quality in your home, and the presence of pets or allergies. Regular filter maintenance ensures efficient airflow, prevents dust accumulation, and improves indoor air quality. If the filter appears dirty or clogged, replace it promptly to avoid strain on the system, reduce energy consumption, and promote better air circulation throughout your home.