That is a question Ive asked myself many times and there is simply not a average so to speak, turtles grow very fast in captivity, slower in the wild but the final size its basicly the same having in acount the turtle is well looked after. The most inportant is make shure you practice a normal water change regimen, a very varied diet including dark leafys, uv ligth and good temperatures and you have nothing to worry about Each turtle has its own growing ways, some do it very fast, others do not