There is no set answer, my 3yr old male hit 7" in the first 6 months I had him (got him tiny(<1.5") from petsmart), he is now only 8". I got my female at about 2" 6 months ago
and she is about 5" now.
it depends what u feed...size of tank and the tempereature..the higher the temp the better..the bigger the tank the better...foods with omega 3 is good for rapid growth