Most shield companies recommend changing when needed. If in the sun a lot they will last a shorter time. We typically see them lasting from 2-5 years like hardhats. Check with the manufacturer. I have long wanted to do an aging study on faceshields but it would be unfair to sit on a roof in the sun since that wouldn't represent proper care. If a large company would like to sponsor such a study, ArcWear does these type of publishable studies for cost of testing.