There are many factors to consider including the length of the dryer vent and how often the dryer is used. In general it’s recommended that you dryer vent be clean once per year.

If the time it takes to dryer close increases over time the likelihood is that your dryer vent is getting blocked.

Installing a bird to guard the covers the vent exhaust will prevent birds from entering the vent.

A gas dryer produce carbon monoxide. The clogged event can prevent the carbon monoxide from leaving the building.

Lint building up in the dryer vents reduces the airflow. Over time the links dries and high temperatures or a single spark can cause they highly flammable lint to ignite.