Carsten Möllering

... just some thoughts - not the "truth" ...

Referring to the - not only martial - arts I know, I consider it to be an important to point out that there is no "mastering" of the art - be it aikidô, chadô or just life. And to second this by pointing out that in most systems shodan is the first student-grade. While kyu ranks are considered to be preparatory-grades.
This answer may be a frustrating. But it reflects the process of learning. If this answer is hard to bear, practice will be.

You mention krav maga: Someone who pracitices for some years will smile about what he thought to be proficient after his first one or two years of practice ... Just like someone does who practices "kung fu".

Driving a car is nothing - except you dont't know how to.
Using a pc is nothing - except you don't know how to.
Binding a bow-knot is nothing - except you don't know how to.
shodan is nothing - except you don't wear one.

Nearly everyone who started aikidô at the same time, I did, got shodan since then. (So when I teach my friday classes there are about 6-8 people. Nearly everybody of them practices about 15-20 years. Nearly everyone is shodan.)
Not everyone of my dohai but still a lot of us got nidan then. sandan there are only few. No one of us got yondan until now. These are the ranks you can reach as a student. And you can reach them by yourself: Practicing, doing a test, becoming next-dan.
Godan you cannot reach by your own efforts. Becoming this first teacher grade will be given to you - or will not. You like or dislike this, nevertheless, becoming godan is different.

So there is a pyramide. Becoming shodan is kind of natural in my context after maybe ten - fiftenn years of practice. Becoming yondan is something special. Only very few reach this aim. Becoming godan is kind of an award.

no

Intensity is just one aspect. I appreciate this very much! I used to practice in the dojo six days a week during my first years. And I practice solo thinges at home every day.

But another important aspect simply is time. Everybodie's body needs a certain time to integrate what has been learned. This time depends on the person, on the things to be learned, on the aims to reach.
I only teach basic movements in my class. And I very often say: "Don't care about the outer movement! Everyone of you knows ikkyo omote. ..." But I teach things like using the shoulder blades in a certain way. Or having "the head reaching into heaven and the feet reaching into earth". ... Or to "not do ikkyo with the arms but with the legs". Or whatever.
I takes time to just understand what I mean. Even for me - while teaching it - I takes time to understand. It takes time to test what is meant, to adjust one's body only here and now to what is meant. I takes time to learn it and to be able to reproduce it every time one wants to. I takes time to make it one's own and to naturally use this movement without thinking about it. ...

I read that fascia need at least (!) half a year to grow and to "adjust" to a certain thing that should be learned. The growing time of muscles most people knwo by experience. ...

Whatever the body has to learn inside needs time, a lot of time. No matter how intense your training is. Your body needs time to develop.

Unlike the grades in the japanese arts, the requirements of becoming bachelor are mostly standardised. So everybody knows, what someone had to fullfill and can judge him or her by the result.

Foremost it does not consider that the requirements of dan grades are not standardised. They are on the contrary very different from federation to federation.

So I think the question how long it takes to get a black belt can be not more than the starting point of a conversation about the learning process of an art.