Looking through our organization’s syllabus and doing some math, the minimum number of classes attended is 450. Having some fun with math, it would take almost 3.25 classes per week to do that in 4 years.
That assumes that you tested immediately upon reaching the minimum amount of classes for each rank, i.e. if there’s a minimum of 60 classes from 4th kyu to 3rd kyu, you tested (and passed) right after your 60th class. Realistically speaking, that’s impossible because tests are scheduled in advance, and not for each student at the exact completion of the minimum number.
The 3.25 classes per week also assumes the student never missed a class for any reason - holidays, vacations, illness, etc.
The minimum number is exactly that, a MINIMUM. Realistically speaking, the CI determines if the student is ready, regardless of the number of classes. I tested early and double promoted my first few belts. I was a 1st dan in an offshoot of my current organization 15 years ago. The syllabus is about 90% identical. The first few belts were more of getting the rust out than anything else.
Most people without any relevant prior experience allegedly test for 1st dan in 6 years or so. My CI does kyu testing in-house, and we test for dan ranks under our founder. In a good way, my CI would rather us be over prepared for the test than under prepared. In his 30 years of running his own dojo, he hasn’t had a student fail dan testing. And yes, more than a few people fail each time (there are students from many dojos within the organization testing together). So I guess it’s safe to say my CI knows when someone’s ready.