Having trained many a soldier, my thoughts on readiness to progress is
-EVERY single person is an individual and grows independently. Some will NEVER be able to handle leadership
-The above comment being said, in the infantry I noticed a rare few are able to handle being MCpl really fast (one of my battleschool peers made it to MCpl in 2.5 years and then a year later joined JTF2 as an Assaulter, so that kind of guy). Looking at the infantry, I would say most should be ready for MCpl (after passing current courses) in 4-6 years. Some may need a little longer. And as I said, some will never be able to handle leadership.
-A pet peeve of mine was seeing people that are eager and willing from trying to progress because CoC get locked into "too new" or "But we have so and so should do his course first because he has been in longer.." I wanted to attend 6B INF PL WO (later 3B) when I had 2 years as sergeant (knowing I would not be promoted for a few years), my RSM said no because X, Y and Z were more senior to me and he was trying to get them on it. X, Y and Z never had the balls to do the course and I ended up passing them anyways. So lose the dogma and bureaucratic thinking
-We are talking MCpl level (and for context, I was infantry), and what we expect of a MCpl and a Sgt are much different. SO I see times were its justifiable to accelerate people to Master Jack but not sergeant
My opinions, take them for that they are worth.