Well, a question for you... do you want it to be so that only one unit can be in region (and counting down to victory) at once, or is it okay if one is in the region and the other enters (both counting down but at different times)? Because I personally would use custom values.
Every 1 sec, pick every unit in (region) and set (custom value) to ((current custom value) +1).
If picked unit's custom value > desired time, then declare victory.
This way, multiple units can be counting down. It also carries along with it other benefits, depending on how you want everything to work...
EDIT: Oh, and don't forget to use another trigger to set unit's custom value to zero when it leaves region