Certain windows take a lot of damage before they are destroyed. Even within the same house some windows break easily (2-3 shots 7.62x51mm) while others are more resistant to damage (9 shots 7.62x51mm).
I'd expect most glass windows to break after only a few shots.
List of affected houses (possibly incomplete):
i_house_big_01_v1_f
i_house_big_02_v1_f
i_house_small_02_v1_f
i_house_small_03_v1_f
i_shop_01_v1_f
i_shop_02_v1_f
Find one of the affected houses listed above and notice the amount of damage it takes to destroy the windows
I noticed that, if you want to destroy a window, it's always quicker to shoot the window frame not the window itself. the connected window will then break in 1-2 shots. Maybe it has something to do with bullet ricochet.
+1. Sneaking up to a window to pot-shot an enemy then finding the glass is damn near unbreakable is a real bummer.
Thanks for the tip, pops, I'll keep that in mind :)