Do criminal trespass warnings ever expire? How does your county manage them after an extended period of years?

No, they never expire, as long as the owner who issued the warnings is still in control of the property. If it is a matter of a posted warning, they are always good.

You have to handle the ancient warnings on a case by case basis. How egregious was the trespass? Did the defendant acknowledge the warnings? Can you prove they were given? How clear, what has happened in the interim, etc., etc., etc.