How long after the last departure shall the airport stay open for an emergency return? For how long after the last take off for that day, will an airport stay open for emergency return of that last departed aircraft? Where can I find that recommendation?

To clarify: an airport needs to be closed during the night due to work-in-progress on a runway, so it can't stay open 24/7 for emergency landings.
Where I work, we have to stay in the tower for 15 minutes after the last departure in case they need to return. To my knowledge, there are no international regulations about this, so the rules may well be different from place to place.