Well, for your first shot (assuming you go first), the odd of guessing a frog is about 5.6%, assuming you won't guess one of the middle 9 squares, as they will be empty. The board is 13x13, making 169 squares; the middle 9 are empty, so 9 frogs will be found on 160 squares, or about 5.6% of the squares.

Running a simulation where both players guess randomly (but won't guess a square that was guessed before; and won't guess one of the middle 9 squares) shows the game is pretty even, and lasts about 144 half-moves.