I look at time in grade standards for post-black belt progression to be ideas. Your art, if you are working on it, will get better with time. Therefore, the idea that 2 or 3 years from 2nd to 3rd is just that you should show that much progression. For some, that might happen in 18 months, for others, it might take 18 years. Normal student training a normal amount and staying dedicated it should take 3 years.
However, we then have the next problem:
Is the "normal" student going to make it to 3rd Black? How about 6th? I would say that once your material learning is finished (3rd Black in most EPAK schools, although you might pick up additional weapon forms at 4th and 5th), further progression can and should be limited by the individual's abilities; certain people simply "don't have it." I'm all for fair progression, and getting people to push themselves and reach new limits. I completely think that ANYONE, with proper time and dedication, SHOULD be able to reach Black. Past that though, I don't want a 5th or 8th degree Black Belt running around who worked really hard to get to a base level of proficiency to be a Black Belt, and then has just kept muddling along since then. They probably won't be able to share and showcase the art as an advanced practitioner, and they shouldn't be someone who is wearing a half-dozen stripes or more.
To the original post:
1) Standards for rank are knowledge of material for that rank and all previous, ability to perform it at an appropriate level (earlier comment about 5 Swords from an Orange Belt should be different than one for a Black Belt is spot on), and knowledge of concepts appropriate to that rank. Time is a guiding factor, but not critical at all. When I've taught underbelts, I keep track of their last test date and actually have a database. I know, that on average, progression from white to yellow takes 2 to 4 months. I had a little system designed that would pop up and tell me that student John has been training now for 2 months. My job as an instructor was just to double check on his progress - how has he been doing and does he need much more material? His name would pop up again at month 4 with the idea being that as an instructor, I should be looking at him as ready to test. If he wasn't, was it him not training, missing classes, etc? Or was it him getting overlooked or an instructor problem? It wasn't a "forced" to test, but it was to make sure students were being watched. It was great at upper ranks of Green and Brown where sometimes time between testing is 10-18 months, and it's easy to think "Oh, John still needs this form, but we'll do that later." And later, with all the students and classes, ends up being almost 2 years.
2) 15 years for 6th is fast. Wrong? Don't know, let me see him, but I would put that one under the microscope because it goes against the time in grade guide. He says he's been a Black Belt for 15 years and is a 4th or 5th, I'd be more willing to accept it. Then again, I go back to my statement that the number of stripes on your belt only matters when you have more stripes than skill.
