I'm on Windows 8.1, 64-bit.

I'm not sure exactly how long it takes, but the overlay crashes after a couple of hours. I thought it might have to do with the number of session kills, as it seemed to have crashed when I hit 100 this last session, but I'm not sure. Achievement pop-ups still display, which is odd, but even restarting the game can't get the overlay to reappear; I have to start a new session in order to fix it.