Yes, it helps.
In fact, it helps double: The dust not only hinders air flow over the coils, but also acts as an insulator!
No numbers, because there are SOOOOO many variables here: The size/area of the coils, their temperature, the speed/volume/temperature of the airflow over them, whether it is forced air or passive cooling, etc. And of course, how much dust (and what kind of dust) is on those coils.
Also, if the refrigerator itself is badly insulated, then it will turn on more often and for longer, so the gain in heat exchange efficiency will have more impact than in the case of a well insulated fridge which is rarely on.