I've read that they can grow up to 14" long and I would assume it would take a few years. It's also going to depend on how much room you give them to grow. If you keep the 7" one in a 55 gallon aquarium it might not grow much more, but if you give it a deeper aquarium (a 75 or 90 gallon) and feed it a healthy diet of live fish, I'm sure it will continue to grow.