I keep several of those on the server and have the automation play one or two at the end of any half-hour that ended early. When I download them from the Ad Council, they have expiration dates listed right next to the download link and I put that in an Excel file which I can then sort by date to see when something needs replacing.
Apparently I am one of the few who bothers, because every so often I get an e-mail from the Ad Council reminding me to remove expired PSAs, along with a list of the most recently ended. I wonder sometimes if those e-mails get ignored by the very stations that need the reminder!
NAB also provides PSAs but very few have actual expiration dates.
Usually it's the stations that have no local sponsors that rely on nothing but PSAs -- they are the ones that let some PSA's run for years. And it's usually the obnoxious PSAs, or the ones that were poorly written and or produced.