i personally haven't kept carpintis before, but a friend of mine has. The female he had grew to about 8" at a casual speed and then slowed right down, it then maxed out at around 10-11" but that took years after it reached 8".
so i would say they are capable of reaching 12" given good water quality, tank size, food and most importantly time. However, they will reach around 7 or 8" and then slow grow from there, so really it depends on how long you want to keep them, as to there max size. if you keep them for life they may end up 12". but if only for a year or two then you may only get 8".
Also, they grow at a reasonable speed in upto the 7-8", then slow. nothing like an oscars growth, more your typical CA species. Not notably slow IMO.
hope this helps, and wasnt too confusing!