As said, depends on the environment you keep them in.
But if you are a good hobbyist like other members of MFK, you can expect up to two inches in the first month, and then a month an inch until they reach 6-8 inches. Then they slow down a hell of a lot. But in time to come, you will look back on these 12 inches beasts and think it was not to long ago they were just 1 inch. They are dfinably fast growers!
Also expect some casualties of cannibalism at their current young age/size. When buying small red bellies, always buy more than you intend to keep in the future.
Unless you get lucky with your lot.
How many did you buy, any picture?