We do have class hour requirements for someone to be eligible to take their rank tests. However, the class hour requirements are but a single prerequisite. Even if they have the appropriate number of hours, you still have to look at their proficiency, their attitude, etc., and see if they are truly ready to take the exam.
The way I see it, if someone can demonstrate the appropriate level of proficiency in the art, and if they can demonstrate that they also meet the physical requirements, then I have no problems with promoting someone to Shodan in a shorter time than the norm.
If there's nothing to be gained by keeping such folks at the ikkyu rank, then why keep them there? If someone has already demonstrated proficiency, has the required number of hours, and has a good attitude, then I'll be more than happy to test them, even if they've only had two years of experience.
On the other hand, if someone has a less than ideal attitude, a less than ideal work ethic, etc., then they won't be taking the exam, until things shape up. Sometimes, this means that it takes additional years.
Then, you have students who learn things more slowly than others. There's nothing wrong with their attitudes; it's just that things simply might not come to them as quickly as it does with your more talented students. They may take 5 or 6 years to get their black belt when others take 3.
I don't see that as a problem, since they're still learning the same matierial, and gaining the same proficiencies. In the end, the finished product is what counts, and if it looks good, then the length of time isn't much of an issue.
Now, all of statements that I have written in this post, are based on the system in which I train and teach. In our system, getting the black belt simply means that you have a solid command of the fundamentals, and have demonstrated such proficiency. In this case, you're ready to take on the more advanced training.
Having a yudansha ranking in our system does not confer any type of instructor status. That's something different...