They also don't know when they missed an offside call, which is really the purpose of the whole "defending team gained possession" rule change. So if a goal is scored, they'll review the tape going backwards until they either see the defending team gaining possession, or the attacking team enter the zone, either onside or offside. Then they'll know if it's a good goal or not.
I would guess in most cases zone time isn't really all that long. The situation in last year's tournament game was definitely the exception rather than the rule, in that we held the zone for a long time before the goal was scored. And even that wouldn't take more than a minute to review to see if the other team gained possession during that time. It's only during a challenge that this looked at, anyway.