I started tkd when i was 8 and didn't get to first dan until i was 19, but that was due in part to schools closing and instructors dying and things of that nature. usually i'd say the average for people with considerable prior experience is 1-3 years (it's only taken me 9 months to get to reccomended black belt in shotokan). my advice to you is to find a school which will allow you to challenge yourself...that doesn't necessarily mean the most traditional school or the most liberal either, just a school which encourages constant growth from even advanced ranks. i have no idea if that helps you at all, but i tried. best of luck!