Let me use an example. Harold Long, Isshinryu, (and a few others I might add) received their black belt in 18 months as stated in their bios. In his perticualar case, being in the military and having military training (and also being of a military mind for protecting ones self) from what I understand spared quite well before he started martial arts (probably). From that point it was a matter of learning technique, kata and control of his mind, body and spirit. My understanding is that there are a number of people who received BB in Japan or on Okinawa in 2 years or less. Respectfully to all..