Matt:The standard says twelve months. When PM equipment was developed, Westinghouse believed they were good for three months in the turn-of-the-century railroad environment. Our valves usually behave themselves for at least six months. Sometimes they'll go the distance for the year. When the temperature goes above 85 F and the compressors are pumping water in the humidity, then they can go bad in a week.Symptoms of a valve gone bad are usually undesired quick action or failure to release.