This isn't a simple scenario. On Diamond, for example, the portal can easily be broken on turn 112. And even on X3DM1, it's often broken on the first turn, although this encounter isn't particularly likely.
Okay, then let's say: interrupting the turn after one of the players breaks through the portal to the opponent's side after turn 121 (insert any date), or immediately on turn 121, if the portal has already been broken by that point.
