Hello,
Yes, that is quite normal. They do grow very fast at that age. On average, they will grow 1/2 inch, sometimes more, per week.
I would guess that he is probably roughly 6 months old, at 12 inches. Give or take 1 month either way. It is difficult to tell when you get them from a petstore, as they don't always know the true age, either.
Maybe you could post a pic so we could have a look at him.
Tracie