Currently, the game lasts till all the frogs are found. However, there's usually no point in playing on if one player is more than 5 * F points ahead, with F the number of frogs to be found. (I say usually because theoretically it could be that all unknown squares could potentially contain a frog, forcing the player ahead to guess - but that's a situation that won't occur very often).

So, I suggest to add another winning condition: whenever you are ahead with a number of points more than 5 times the number of unrevealed frogs, you win. This will shorten many games by dozens of moves, where the player ahead is going to play frog finder, not caring who reveals the frog.