There is really no right answer but, generally speaking you should never go more than one year without cleaning your dryer duct. If you have a large family and a long dryer vent twice a year may be necessary. It’s very important that when your vent is cleaned that the technician gets to the vents exit. The only way to test a dryer vent is to turn the dryer on and go to exit to measure the airflow. Ideally this should be done using an anemometer.