Dryer vents are a leading cause of house structural fires. Over time lint from the clothes builds up inside the vents and may become a major fire hazard. These fires can be prevented by having your dryer cleaned on a regular basis.
After using your dryer for a while you get to know how long it takes to dry a load of clothes. If you notice an increased drying time the probable cause is a restriction within the vent. The venting should be cleaned.