While not Shotokan, our system is laid out to reach 1st dan in 3 years, about one to reach 4th gup and 2 more to go on to 1st dan. It is also stressed that time at a given rate (2 classes per week-2hrs each class =16 hrs/month) along with techniques are minimums. However, this is also subject to instructor discression-with approval from our grandmaster. Within our system, it is feasable to reach 1st Dan in a year, but that student would be attending double or triple the normal 4 hours per week. I would therefore suggest clarifying the time spent training in hours, not just weeks or months. Depending on who is teaching at any particular time, it has been possible for our students to attend 5 or more classes per week, attending his own instructors class as well as other instructors in our association and our grandmasters classes. This student might attend say 2-3 classes in this town, then drive to another 30-40 miles away and attend another of our associations classes on different nights. I am not aware of anyone who has done so. I am aware of some of our exceptional ( good kids, 3.5-4.0 students at school etc) students who do put forth maximum efforts to attend 3-5 classes per week on a regular basis and learn the techniques well, practice at home like they should and advance thru the early ranks, or pass a rank, say from 10th gup to 8th gup or 7th gup to 6th gup. I am not aware of any of our students recently (there are some-2 or 3- I have heard of, very rare, that went to 1st Dan in 1 or 2 years) that have advanced at an accelerated rate once they have reached 4th gup (brown belt)
Once first Dan is reached there is a different progression, 1 year for each additional Dan rank, so 2 more years (total 5) for 2nd Dan, 3 more from there (total 8 years) for third etc.
I think the bottom line is how talented is the student, how much do they train and who is their instructors.