Hmmm... that's a tough question. For the very best of runners, it seems to be 100 miles/week give or take 25ish.

For the average Joe who is trying to get better, I think that about 30-35 miles per week is a good goal. In my experience it seems to be the point where you can get all the quality running in that a person would need and you see a lot of "return" on the investment. I think beyond the 30-35 mile per week range up to about 60 miles per week you continue to see a good, but decreasing, return on investment.

Under 40 miles per week, I usually recommend a long run of 25% of total weekly run volume. A typical week in the 30-35 mile/week range for me would be something like:

Monday 6 mile run
Tuesday 7 miles - 2 mile warm up, 3 minute repeats "hard", 4 times with 3 minutes rest/light jog in between each repeat.
Wednesday rest
Thursday 2 miles - 1-2 mile warm up, tempo(pretty hard) of 15-40 minutes, cool down if necessary
Friday rest
Saturday 9 mile long run easy
Sunday 4 miles easy

32 total weekly miles. Long run of 8 miles. 2 "hard" days, one with a shorter duration/higher intensity, another with a longer duration/lower intensity but still hard. A couple of easy days.

When increasing mileage, take it slow and easy. Build up a few miles per week at a time.