I strongly disagree with the concept of having a minimum required time to obtain a certain rank. I understand that it is there so that the student can get a certain amount of experience with the techniques of the current level before moving up, but each individual learns and obtains experience at their own speed and if they are able to demonstrate their knowledge and ability quicker than the average person, they should be allowed to test quicker than the average person.
I took Tang Soo Do for approximately two years and obtained my green belt extremely fast because I had seen all of the forms before in Goju-Ryu Karate. I was allowed to get the rank quickly because of the experience in a similar art, but after I started moving into the unfamiliar territory the whole minimum time requirement kicked in.
When it comes to the martial arts, I am fairly quick. If I have a healthy body and mind, I can pick up a technique or form after just watching it once or twice. I know how the body moves and by using my imagination I can experience the technique or form and get a good grasp of it without ever having practiced it. I might require a few minor adjustments, but overall I've done pretty good. So I would say that I've got the ability to advance quicker than the average person.
In this Tang Soo Do class, I was just this little two stripped green belt who had all of the red belts and some of the black belts coming to me to ask them how to do their forms, one-step-sparring, and self-defense techniques. I had all of the required information memorized up to around 3rd Dan. I was being held back because of this time restriction. Of course, I was still learning the advanced techniques so not many will understand why I feel I was held back... take note that I say I memorized the information up to 3rd Dan. Memorized is very different than internalized. I've internalized all of the techniques up to the 1st Dan so that even now, several years after I've last practiced them, I still know and can demonstrate them. The techniques from 2nd and 3rd Black I haven't memorized... Primarily because I wasn't allowed to practice them in the class where they could be internalized.
I now teach Dragon Kenpo. I don't have any set requirements for belt promotion nor do I even test a student for rank. When he/she demonstrates they have the ability and knowledge to move up, they are moved up. They might provide a demonstration to prove to themselves that they deserve the rank, but there is no test.