All labs are set to close on a timer which is located in the bottom left hand corner of the lab screen. Once it reaches 15 mins you will see a warning that will enable you to extend labs for another 60 minutes. You can do this as many times as you like. However, once the timer reaches 00:00:00 the lab will be entered into a saved state automatically. Labs will only be saved for a period of 4 to 7 days depending on the lab environment.
Should you click ignore by accident, simply click on the Orange Timer and the pop up will reappear.