9500 kobans spent, approximately 1000 battles, 26 gifts received, no girl
but hey, at least the game tells you you're a winner everytime you annihilate that ninja spy!
I stopped at 9,5K kobans, which in combination with 4 days of active trying equals about 1000 battles. So I guess the drop rate is less than 0,01% but I can't even care anymore, although I do feel sad for the people who spend real money on this