The only time Hannah chews on her kong is when we are playing fetch with it. Before she relinquishes it back to me, she has to stand there doing the chewing motion. Other than that she really doesn't chew on it at all. What she does enjoy (although as we are dieting we don't do this much) is my putting a small milk bone inside the kong. She then takes her nose and pushes it in different positions on the floor until the milk bone falls out.

I think a lot of dogs lose interest in chewing as they mature. Mine have
(unless you count my chewing my undies).