What you're looking for is the TIMEOUT portion of the trigger. It works in seconds, min max and med should be self-explanatory, and should be what you're looking for. Also make sure the trigger is set to Repeatedly. This is how it works, as far as I've been able to figure it out:
A) The trigger is activated by whatever is in the Condition field (probably "This" in your case) : meaning any BLUFOR inside the trigger is detected by any OPFOR.
B) The Timeout begins ticking down.
C) The timeout ends (I believe) if the Condition is turned to False before the timeout runs out. If not, whatever is in the On Activation field is activated (for instance a variable is set to true that alerts the whole town).
I suggest you try it yourself : place down the trigger and the enemy patrol, and give the trigger a certain timeout (say, 30 seconds). Place a hint of some kind in the On Activation field so you know if it's been activated. Place another hint in the On Deactivation field just to test for it. Now go on a killing rampage, and see if the trigger is activated if you kill the enemies before 30 seconds have passed. Play around with the seconds and such, and see what you can get.
Wolfrug out.