How long is a clearance void time valid?
Since you didn't state jurisdiction, I'm assuming US.  

From the AIM:  


  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.


The requirement is to be airborne prior to the clearance void time. You need to contact ATC within 30 minutes of the void time either in the air or to notify them that you did not depart.