honestly I'm pretty sure it just depends on diet. I know they can grow really fast but it's not really something you want to do, because this is caused by over excessive feeding.
Pet turtles (who grow faster than those in the wild because of the rich diet) will often grow to around 4" (give or take an inch) long in their first year. After that they grow more and more slowly every year.
*Kiki & Riri ~ 2 Female RES 5' - I lost Riri * 2 Yellow Bellied Slider - One left * 1 Chinese Golden Thread Turtle - Lost *Yellow ~ Female Malayan Box Turtle *Macho Kitty ~ 12 years male DSH Cat - RIP Feb 2012
ok, that gives me plenty of time to save up for an upgrade, which i hope to make few. go from a small tank to a hopefully large one, not the minimum. i plan on spending as less as possible, (fewer upgrades.)