At about 3-3.5 years old, mine are about 6" 6.5" or so inches. I expect them to grow about .5 - 1" per year after this. The larger the fish, the longer it may take to reach the maximum theoretical size. 8" is possible, but I'd imagine that 8" fish will be quite old. My guess about 4-5 years old provided he's well fed and in a large enough tank.
Females remain a fair bit smaller at around 4.5" - 5".
Hope this helps.