They seem to vary a bit, but most of mine seem to slow down a bit at 4 months, and by 7-8 months have 90% of their length. After that they grow more slowly and fill out. I haven't measured specifically for that, and its just my observation.
I've also seen that leopard geckos from pet shops will grow surprisingly fast after they settle in, and believe this is because they were underfed (often due to stress more than lack of food). When they are set up properly, they make up for lost time .. so its especially important to supplement their food with calcium and minerals.
-----
Andrea A. 