Not very quickly. It was hard to find a reliable growth rate estimation. They tend to grow in an "S" curve from what info I could find, where the first 2-4 years of their life they grow very slowly, possibly only 1/2 - 1 inch per year in length. However once they pass that, they grow relatively faster, getting to breeding age at around 6-8 inches, which can take many, many, years, and then grow slowly again after that. I guess, in short, they don't grow very fast at all. Certainly much, much slower than something like, lets say, a sulcata.