I doubt there's any set time. It very well could vary from location to location. (At airport locations, when you've entered your arriving flight number into the reservation, it's until some amount of time after that flight actually lands, not when it's scheduled to land. But obviously that doesn't apply at a non-airport location.)

Can you call the location now and either (a) ask them, and/or (b) tell them that you may be late for that reservation and to please hold it until XX:XX?