If you shut it down for too long and there is a lapse in reopening it, planes are grounded for an extra bit of time.

If you shut it down for too short and there is a lapse in extending the grounding, planes are getting shot out of the sky (or whatever threat it was).

edit: I would add that maybe there are forms for shutting down airspace of various specific time lengths and a convenient time for something of unknown duration would be 10 days. 10 days might also be enough time to be sure whatever resources need to be brought to bear on this are available where an hour or day might not be. Shut it down basically indefinitely, or at least long enough that the crew who handles this extraordinary situation will be on hand to turn it off.