I have never used this, so I do not know if it is based on:
Time from mission start UNTIL Fence IN
or
Time the Player spends INSIDE Fence
Either way, DEBRIEF statements do not control any actions within the game, only REPORT outcomes.
To control ACTIONS, you must use the EVENTS, and FENCE-IN time is NOT a condition that can be used.
To achieve your purpose, a MOVING AREA event would be better. Place a unit 'on the FENCE' and set a FLAG when the PLAYER is very close (say 1 nm). Then, set a TIME EVENT to trigger another USER FLAG 1 minute (or whatever time you want) after that FLAG is set. Use the second flag to trigger your enemy aircraft to take off or attack the PLAYER.