My average seems to be once per 10k+/tire on the variety of reinforced tires, i.e. longer than the lifetime of a tire, consistently with having yet no flats on Supreme. For serious glass spills on my route, I actually carry a tent whisk and pent set, following the logic that it makes more sense to remove the glass with the pan than with your tires - there are parts of the route where the spilled glass has nowhere to go and will tend to stay put within the boundaries of the pavement. The bottles seem to be thrown out of the passing cars.