The once-a-year rule
Most households should have the full vent run cleaned yearly. Large families, long vent runs, and vents with several elbows need it more often. The lint screen you clean each load only catches part of what the dryer pushes out.
Warning sign 1: two-cycle loads
When a normal load needs a second cycle, hot moist air is not leaving the house. The dryer works twice as hard and the clothes still feel warm and damp.
Warning sign 2: a hot laundry room
If the laundry room gets hot and humid while the dryer runs, exhaust air is leaking or backing up into the room instead of leaving through the wall.
Warning sign 3: the outside flap
Stand outside while the dryer runs. The vent flap should open wide with strong airflow. A flap that barely moves, or lint packed around the hood, tells the whole story.
DIY or pro?
Short, straight vents are a reasonable DIY with a brush kit. Long runs, roof exits, and vents with elbows hide lint a home brush misses, and a pro cleaning measures airflow before and after.