The 10 years to 6 dan MJER I believe refers to one case, an American who lived in Japan for 10 years, studying constantly under an 8th dan, with periodic workouts under a 10th dan and a Soke, training four days a week, walking home swinging suburito and scaring the locals. I do not believe this is typical even for Japanese nowadays.
Agreed. I do believe I used the term, as little as 10 years, but I don't think it was clear enough. My post did make it sound like 10 years was not uncommon. My apologies for not being more clear.