I always heard that they grow for about two years.
Based on my own frogs, I had one male that grew for two years. However, I don't know how old he was when I got him. He was about 2.5''. Now he's just slightly under 3''.
I have another I got at six months of age who was tiny. It took him about four years to reach 2.3'' and he seems to have stopped growing.

So who knows?