IIRC, novices and accepted generally spend about a decade in each rank, twenty years in total. We see that women who climb the ranks faster achieve the shawl before they've reached their full potential.

It occurred to me that the average length of time a girl spends as a novice and as an Accepted could be a result of the Aes Sedai's hierarchy system. If girls were promoted to Accepted or the Shawl before they reached their full strength, it would mean that they would spend the next few years slowly gaining in rank and influence, and that the sisters they interacted with would have to keep in mind both their potential and their actual strength in the Power.

I doubt it was ever a public decision, or even an active one. Rather, I think the time girls spend in gaining the shawl is a function of their training and the mindset of the Aes Sedai in power at the time.

I would assume that during the Trolloc Wars, when they trained girls hard and fast, more girls were forced to reach their full potential more quickly, and were promoted more quickly as well.

During Hawkwing's siege, we would probably see a similar effect, because Aes Sedai outside the tower were dying, and those inside the tower had nothing else to do but train younger girls, so they probably progressed more quickly than girls in times of peace.

The Aes Sedai in power during those times wouldn't be consciously thinking about where the upcoming girls would fit into the hierarchy, but they would have that ranking system in the back of their minds when judging if a girl was ready to be promoted.

Just my random thought of the night.