Exactly how quickly a fish grows depends on circumstances and cannot be accurately predicted. Silver sharks actualy grow rekatively quickly and should be placed in the right size tank to begin with as they are nervous and active and, as they need to be kept in groups, won't work in a small tank. Considering they are usualy purchased at around 5" in length, it won't be long at all before they require a 125 gallon. If you want a fish that grows large, be prepaired to put in the right size tank. if you don't have that tank now, wait until you do have it. It's realy irresponsible to buy a fish knowing full well you can't accomodate it's full size. I appreciate you may be planning to up-grade but planning is often not enough.