Average is about 6 to 12 inches in the typical community tank, before they break their back and die. Keep them right and they might get around 36 inches. (More typical in the 25 to 30 inch range)
i have had mine since the 7" or 8" mark. but now hes at 16". i have him in a 75g for now and working on an upgrade. his final tank will be somewhere between 300 and 500g