Many times the rates are nowhere near this number. For every 30 images taken I would estimate that you might get 2 good images. You cannot really tell how good a flake is until you get it under the microscope. Even broken flakes can have some interesting parts on them, so I have a 10x objective that I swing into place to see if I cannot get an interesting image out of a bad flake. I have a lamp post in my front yard that I can quickly look at night to see if it is snowing and what type of snow is falling.
Hard snowstorms have too many broken flakes and bits to bother going out and photographing.
The most desirable storms are when it is just barely snowing and there is no clumping of the flakes yet. Many times a storm can have windows of different types of flakes. If you observe a snowstorm and decide that the flakes are not desirable, just wait 20 min and look again. The snow forms can change quickly and you must be ready to act quickly. This waiting and hoping for good snowflakes can lead to sleepless nights.