I asked this question recently to one of the head press installers for Man Roland. So here is the answer. It's time to change your pins when they are either damaged or worn. Sounds simple, but it is apparently a little more involved than that. If you work on a press folder that uses pins, you may be interested to know the three reasons he outlined for changing them. They can help in troubleshooting folder problems.
1. They get damaged. This is when most of us pressmen change the pins. They get bent or break off in a folder jam. Even then, if you are like me, you will not change the pin unless it is really bad.
2. Worn beyond bevel. This is a less noticeable reason to change the pins. Quite often at the tip of each folder pin, there is a small bevel that leads up to the point where the pin is meant to pierce the paper. As time goes on, this bevel will wear down. What makes it less perceptible, is that the bevel can wear down with it. Once that bevel disappears, it is time to change the pins.
3. Worn down. This is another reason to change the pins. It is closely related to the previous one and is also not so perceptible. Over time the pins wear down. What makes it difficult to tell, is that they wear evenly (depending on the products you are running). A measurement of the height before and after will reveal whether it is time. If you lose any more than 1/8 of an inch on height, change them. Probably crooked cross folds are a good indication of this.
The Web Press Troubleshooting Guide
Web Press Troubleshooting Guide - Crooked First Fold