I provide the scientific answer of, "Gee, I dunno".
BUT the question made me curious too, so I did a little calculating. I typically replace them after 300 to 400 miles (yes, I keep track). Last year, I ran just over 1600 miles (YES, I DO keep track), so that nets me about 5 pairs a year. Sounds about right, I'm going with 5 pairs. Gosh, that's like a new pair of shoes every two and a half months! Mom should be proud.
In somewhat-related-but-kinda-not-really news, Jack Lalanne died over the weekend at the ripe age of 96. I for one can remember turning on the old black and white TV from my childhood to find Jack Lalanne getting fit and motivating the viewers to get off their butts. Yes, there was life before "Biggest Loser". Nice guy, that Jack Lalanne.
May he rest in peace.