quick up to 6-7 inch (roughly a year if they start at 1.5 inch.)
Thereafter growth really slows down, say 1/2 inch per year, especially with AT. Very very few silvers are seen at 12 inch in aquarium. Mine i bought at 7 inch, and it grew about 1/4 inch in a year.
But keep them in brackish all their life and maybe growth will be a bit quicker.