22 years for me. But in the beginning of my flying "career" I never cared about loyalty, just cheap flights. I just happened to be lucky to go from Northwest to Continental to United, being a EWR hub flyer. Biggest decision was to convert from DL to CO after merger.
Now, I average 125K a year, so the next bump is 8 years if I maintain this, 10 or more if I slow down a bit.