That's like asking someone to predict how fast their kid will grow til he/she is full grown.
There is no way to accurately predict when it will reach full grown. Growth rate is dependent on a number of factors.
Genetics of that specific animal
and, while this doesn't apply if its in a tank alone, most animals will also factor in predators and proper habitat.
If conditions were perfect, growth rate would be quite rapid. In a pet store environment... typically you won't find perfect conditons in even the best of those places. (I say this with many yrs of working in pet stores and dealing with them for first hand experience) It could take 6 months, it could take a year, it could take 2 yrs... only time will tell.