How often should the vent ductwork be cleaned, and why does it matter?

Vent ductwork should be checked for lint buildup and cleaned at least once per year. If a noticeable reduction in drying performance occurs, check the ductwork immediately for obstructions and blockages. Damaged or restricted exhaust systems are not covered by the dryer warranty, and damage to the dryer caused by inadequate exhaust systems is also not covered. (Page 25)