DIAGNOSIS Extended dry times are almost always a heat flow problem. The dryer is working, but not getting hot enough, or the hot, moist air isn't escaping efficiently.
FIX Run through this checklist. 1) Clean the lint filter thoroughly—a nearly invisible film can restrict airflow. 2) Check the external vent flap outside your house. Go outside while the dryer is running and make sure the flap opens strongly and you feel a robust blast of warm, moist air. If it's weak or the flap barely moves, you have a vent blockage (see the OE fix above). 3) Make sure you're not overloading the dryer. Clothes need room to tumble for proper drying.
IF THAT DIDN'T WORK The heating element or one of its associated components (thermal fuse, thermostat) might be faulty. This is less common than a vent issue, but if your vent is clear and airflow is strong, a lack of heat points to an internal electrical repair.
HEADS UP Running a dryer for hours on end with a restricted vent is a major fire risk and wastes significant energy. Don't just keep restarting it. Diagnose the airflow first—it's the cause 90% of the time.