The second TKD club I was involved in had an expected 5 year time-in-grade to black belt, for administrative reasons. It is populated primarily with children and teens (up to college age), with very few settled adults involved as students.
Thus, the class format is tied to the rhythms of the school calendar, with belt testing done at the end of each semester. With 10 Geups to get through (white->yellow->green->blue->red, with intermediate stripes), this will take 5 years, unless you also rigorously attend the reduced schedule classes during the summer, which few do, so there are often only a handful testing after the summer 'semester'.
I remember being 'bored' with the yellow belt curriculum over the course of the better part of a year, but that was mitigated in that the material being trained was generally applicable to all levels, rather than 'special techniques' for each belt level. That is, rather than working on memorized sets of pre-arranged stuff which are belt segregated, everyone would work on the same drills, or combinations, and the higher ranks were simply expected to perform at a higher level or to perform a more difficult/extended variant of the exact same combination.