2" per month is probably a good average, just remember that when they are really small they have very fast metabolisms, and the larger they get the slower their metabolism gets. A 1" rtc will need to eat more frequent and larger meals for it's size than a 1' rtc, and once they hit 3' they never need feedings more than once per 5 days unless you are feeding really small meals. They grow very fast to begin with and as they get larger the growth becomes slower and slower. From my expirience 3" a month if you get them super small under 2", and 2" as an average after that until about 1', and then it's like 1.5" till they hit 18" and about 1" after that and at 30" they slow way down. You'd be lucky to see .25" per month, and at 36", your looking more at .1" per month, do the math they grow continously their entire life, but it will not be noticible month by month after a long time.