It's variable. I have chicks that have different tail lengths at different ages. The stages of growth are not the same between individuals unless there's a very high degree of inbreeding. Often you don't know for sure until they get their adult feathers.
Tails are something I've been focusing on with mine because
I have a type I really like (more fanned and full looking, which may or may not be to standard) and one of the original roosters passed down a longer tail... offspring that take after him often look almost like little Sumatras for a short while, then it comes back towards normal as they grow into them. Ameraucana do have Sumatra blood back there, and honestly I wouldn't mind a really long tail to show up for a project