In Ji Do Kwan it took me 4 years or hard trainin to get my black belt 1st dan, on Hwarang Tae Kwon Do it took me 3 years to get my second dan black belt.
Now talking of my actual dojang (HTKD) for the average Joe the time from white belt to black belt is 3 years, it's enough time? it depends, it's not the same the student who goes 3 times per wek (e hours per week) that the student who trains 5 times per week, however I think a well comitment student who trains regulary must at least be in TKD classes for a period of 3 years to get his red belt and then a fully year of red belt to become a black belt.
Back in the 80's in JDK thta was the norm, the student who train regulary, atended to tournamnets at least once or twice a year for a period of 3 years he will become a red belt tenh a fully year to make his tesis, 100 hours minimum of social service (assisting sambonim with the class/ teach class, helping with some chores like preparing things for exams, tournaments,etc,etc.)
This times sems sambonis rush the students to next levels so achieving black belt status is about 3 years.
Manny