Decently frequently, about 1 for every 2 cheese. It was mostly an issue because A. the bombs and cheese would fall so fast I could rarely tell which was which before they hit my box, B. Bombs undid 5 cheese points, so if I got unlucky all my progress would be quickly wiped out in just 2 or 3 unlucky hits, and C. It seemed like bombs would spawn directly over my box quite frequently, so if I didn't react immediately (which I often didn't due to A) I would lose 5 points instantly.
I think most of these would be solved if the box was near the bottom of the screen rather than the middle, and/or if you had more time to react before the bomb/cheese reached you. As it stands the punishment feels too high for how hard it is to react imo.