Actually, the only accurate answer is: as long as it does take. As an example: suppose I have two students, both with comparable physical skills. One of them is in the school every night and works out on weekends, etc. The other, due to prior committments, can only attend two classes per week and gets in some occasional weekend workout time. Both of them have an excellent attitude and are dedicated to the concept of Black Belt Excellence. One would think that student #1 would achieve his rank first, and he probably would. However, he wouldn't get it too terribly much before the other student, because becoming a Black Belt is not just about punching and kicking and doing forms and sparring and breaking boards. There is an internal growth that needs to happen and until I see that growth within the student, I won't promote him. As I have said many times: you don't earn the rank. The rank earns you. When you are ready to be a Black Belt of any degree, you'll be one and not before.