I think that the mileage flown matters much less than the number of journeys.

A passenger commuting once a week from London Heathrow to Amsterdam would achieve 20,745 miles over 90 flights (I assume that they get some holiday!) while a passenger travelling twice a year to Sydney would fly 21,146 miles.

The former would clearly be a frequent flyer but the latter would, in my view, not be.

I reckon that if I flew one return trip a month then that would make me a frequent flyer. 12x CE trips yields 960 TP and Silver status while 12x CW trips yields 3,360 TP and Gold status with some further benefits, which seems a reasonable reward for such frequency of travel to me.