The frequency of purging a truck compressed air dryer can vary depending on several factors such as the type of dryer, the air demand, the operating conditions, and the manufacturer's recommendations. Generally, the recommended purge frequency for a truck compressed air dryer is between every 20 to 30 minutes.

However, it is important to note that over-purging can waste compressed air and increase operating costs, while under-purging can lead to moisture buildup and damage to the air system components. 

Therefore, it is crucial to follow the manufacturer's recommendations and to monitor the system regularly to ensure the proper functioning of the truck compressed air dryer.