On another thread some times ago, someone (CWS if my memory is good) computed that based on the average number of flight of a UK citizen, you would be a frequent flyer if you do 4 trips a year.
I agree that mileage does not matter so much as number of trips. I do around 60k miles a year, but around 80 segments as I mainly fly short-hauls (2 to 4 long-hauls trip a year). As I found myself at my home airport on average three times a month waiting to depart for a trip, I would consider myself a frequent flyer as I am there with a certain regular frequency.
Still, if you're in an airport for more than one O&D return per month on average, it's not a bad definition, because you're interacting with all the apparatus of the airport and airline even if you're not in the seat itself for that long.
Agreed, I would put the threshold at 2 return a month though.