max size in captivity is in the 18"-24" range depending on a number of factors, like diet, genetics, environment etc etc...
growth is fast when the fish is young (say under 12"-13"). sorry i can't provide specific numbers like how many inches per month. maybe others can help.
between 13"-16", growth slows somewhat compared to when the fish is younger, but still pretty decent. after this, growth slows down further as it inches toward its 'final' size, and the sooner you start noticing a drastic slow down, the smaller its 'final' size is likely gonna be...
typically i would expect an individual to be able to reach at least 18"-20". whether it can get to 2ft will likely depend on the genes and how aggressive it feeds etc. and it will take a long time.
NOTE: what i referred to above is not under any power feeding etc.
EDIT: i didn't see your reply when i posted Cohozard. i seriously don't think it'll reach 30"+. and not in 3-4 yrs. maybe in the wild. 2ft is more like it, imo and in what i've read/seen.