It doesnt take too long for even a little one to grow close to 2'. Mine was probably in that range after about two years. After that it has mostly just gotten fatter. The biggest concern for me now is avalanche proofing the tank. At this size its pretty strong and can knock over small rocks and unattached coral easily.
Here is a good article on Morays
A Serpent For Your Reef Tank:
A Look at Fish-Safe Eels
http://reefkeeping.com/issues/2002-12/fm/index.php