Don't know about an average, but I just went and took a look at several of mine. They range from 3-1/2" to almost 6" depending on the rifle and scope. My bolt actions run further apart than my single shots. The single shots run 3-1/2" to 4-1/2" and the bolt action are 5" to 6". Hope this helps.
It doesn`t reall matter as long as your can get the scope mounted where you want it. I`ve had them from just far enough appart to get the adjustment knob between to over 1 foot appart on different guns I`ve had, and they all worked.