What the colours mean, how long each takes, and why nobody in the dojo makes much of them.
Belts are a progress marker, not a ranking of worth. They exist so a student can see how far they have come and an instructor can see what to teach next.
Grades below black belt are kyu grades, and they count downwards — 10th kyu is a beginner, 1st kyu is the grade immediately before black belt. Grades from black belt upwards are dan grades, and they count upwards.
So a student goes 10th kyu, 9th, 8th and so on down to 1st kyu, then shodan — first dan, black belt — and then counts up again.
Early grades come quickly — often a few months apart — because early progress is fast and visible. The gaps widen as you go up. The step from 1st kyu to shodan is measured in years, not months.
Nobody is graded on time served. You grade when your instructor judges you ready, and the grading itself is the test.
Kihon — the basic techniques. Kata — the set forms. Kumite — sparring, appropriate to your age and grade. Fitness. And, from a certain point, the ability to keep going when you are tired, which is much of what is being examined.
Junior gradings are scaled to the child. A seven-year-old is not doing what an adult brown belt is doing.
It is the oldest line in karate and it is still true. Shodan means a student has the basics in order and can now start learning properly. Above it the dan grades continue — and in MOKNZ they continue to eighth.
Which is the honest answer to how long a black belt takes: several years to reach, and then the rest of your life.