I second what Tom said. My RT is about the same age is yours and he has 5-6 inches. I have heard some people say that a good rule is the length of their body times two.
Thanks. He has several inches of substrate. He made good use of them his first night. It took quite a bit of poking around to find him this morning to get him into his soak.
Awww!!! So cute!!! My little guy went nuts the first few days that he had the coco coir. He definitely was one happy little guy! He still digs down every night and he barely ever uses his hides.