Some really great responses. It comes down to the individual really, some people just take longer. It took me 7 or so years just to reach 4th Kyu, and that was by choice. I wanted to take my time and really absorb and understand each grade, and only wanted to do about one grading a year (plus, the gradings were brutal... didn't wanna do that more than once a year XD).
But then I've heard of people as live-in students, training almost every single day for many hours, and reaching 1st Dan within a year or so. At first I thought that was far too fast, but seems it may have been more common in the older days (from the stories I've read). And when you think about the quality tuition and quality of training they're constantly exposed to, it's no wonder that their skill and ability just skyrocket...