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...
Time is more important than distance. Engine speed will be the big variable in this, but in general the thought process goes something like this:
Time to start times the current drawn by the starter motor gives you the energy consumed in starting (more or less). Say your starter draws 240 A and it takes 15 seconds of cranking to start – that's 240 A x 1/240 of an hour or 1 Amp-Hour.
Alternator output times running time gives you the energy replaced. Lets say your alternator is putting out 60 A and you run the engine for 1 minute, that's 60 A * 1/60th of an hour or 1 Amp-hour.
Using those numbers, in a perfect world, it would take 1 minute to restore the energy consumed in starting the car. Charging isn't 100% efficient and some of the alternator output is being used to run the car, so you may need more like 2 or 3 minutes (or less if you have a high output alternator).