I have been replacing PCV valves for many years as standard maintenance. I have never had a problem with one until this past year when my car started stalling at lights. Since I never had a problem due to PCV valves, I never suspected it. I did a lot of expensive repairs before the problem was identified. My answer to your question is to replace them regularly - every 60,000 miles, and also replace it if you start having idle problems. It is cheap and easy insurance.