In BJJ, 2 years per rank and 10 years from white to black belt seems to be be considered a typical course of promotion. BJ Penn earned the nickname, The Prodigy, when he progressed from white to black belt in about 5 years and won the World Championships (Mundials). Following is some cool video of Penn as a white belt beating up some judo black belts.
So now here's a question for you. Should jiu-jitsu students be required to be as good as Penn was when he got his black belt in order to get theirs? I say not every student should be expected to be able to do what 'The Prodigy' did (There's a reason they call him that). To require them to be that good just to get black belt would lead to some ridiculous rank inflation. here's what I mean...
I've heard that many military expats in the previous generation took 6-9 months in Japan or Korea to get their black belts, then they returned to the U.S. and started teaching. What if some of these guys, not wanting to look bad to their teachers, set up requirements that it should take about 2 years to get a black belt. So, the second generation actually ends up better than the first generation when they receive their black belt. Now imagine a conversation like this...
"How long did it take you to get your black belt?""Oh, My teacher was rough on us. It took us 2 years!""Well, it took us 3 years.""I heard that fella over there makes his students practice 4 years to get their black belts! They must be really tough!"Everyone then thinks to himself, "I guess I'd better make my students practice 5 years so we can be the toughest."
Pretty soon it takes 20 years to get shodan! How many people have heard folks bragging on internet forums, "It took me 12 years to get my shodan!" This is like making everyone suffer for someone else's great performance just so folks won't think your martial art is too easy. This is ridiculous when you consider that black belt does not represent expertise - it is just the 'first step'.
Standards are necessarily minimal standards. You don't see medical or engineering (or any) schools making each class have to be 50% better than the previous class just in order to graduate. If you did, soon there would be no graduates.