I thought I heard the guy at the store said 7 miles, but it could have been 7 minutes. What is average? I live about 4 miles from work and it takes around 10-12 minutes to get there depending on the stop lights.
There's too many variables to come up with any one particular number. It can't be too very long, what with cars that shut themselves off and then restart at each traffic light. :-) Also, note that today's alternators are MUCH more efficient at idle RPMs than the generators of years ago (some of which would discharge rather than charge at idle). I suspect that the real number is somewhere in the range of double digit seconds...