Air ducts should be sealed whenever there are signs of leaks, gaps, or disconnections in the ductwork. Additionally, it's advisable to inspect and seal air ducts during routine HVAC maintenance, which typically occurs every 2 to 5 years. However, the frequency of sealing may vary depending on factors such as the age and condition of the ductwork, the climate, and the presence of pests or rodents. Regular inspections can help identify and address any issues promptly to maintain optimal efficiency and indoor air quality.