Once it reaches 20" it'll slow down loads and fill out , like turtles said prob max out at 24/26" in a 180 , mines in a 180 he's six yr old and prob 22/23" atm still filling out slightly , mine was in a 4x2x2 for about 3yrs and was fine no probs
I had a jar in an 8x2x20". In a years time, it grew from 3" to 14". Diet started off as MP, then eventually pellets. The bigger the tank, the faster and bigger they grow