Já que você não declarou jurisdição, estou assumindo os EUA.
Do objetivo:
1. Clearance Void Times. A pilot may receive a clearance, when operating from an airport without a control tower, which contains a provision for the clearance to be void if not airborne by a specific time. A pilot who does not depart prior to the clearance void time must advise ATC as soon as possible of their intentions. ATC will normally advise the pilot of the time allotted to notify ATC that the aircraft did not depart prior to the clearance void time. This time cannot exceed 30 minutes. Failure of an aircraft to contact ATC within 30 minutes after the clearance void time will result in the aircraft being considered overdue and search and rescue procedures initiated.
O requisito é estar no ar antes do tempo de folga. Você precisa entrar em contato com o ATC dentro de 30 minutos do horário nulo no ar ou notificá-los de que você não foi embora.