average growth for silver aro under 24" is 1 to 1.25 inches per month. There are case of both quicker and slower growth. Being you silver is 1.5" I would epxect about 3/4" over the next couple months. I had two silvers at 3.5" in 4 months time one was 7.5" while the other only reached 6.5".
My silver was purchased 3 months ago at 6", it is now over 12"!!!! NO KIDDING!!!! Now, my h20 quality is really good, and he eats A LOT!!! I vary his diet almost every day (live minnows, shrimp, beefheart, live crickets, Hikari bio-gold and hikari food sticks, but just in the last month it has started pounding the hikari sticks and pellets. Just got a baby jardini (of course in another tank) around 2" and havn't seen any growth yet. The Jardini eats same diet with equal enthusiasm. I firmly believe in varying their diet. I have much healthier and happy fish, that usually grow like champs when I give them a mixed diet and keep your h20 parameters within check ALWAYS! GOOD LUCK!!!