a maybe a few months depending on how big it is when you get it. Dont try it though because I cant begin to tell you how many people buy those planning to upgrade. If you can afford the upgrade now than to be honest you probably wont be able to when he outgrows the tank.
Depends on the dimensions. If its a 4 footer, than you could keep it in there comfortably until it reaches around 12-15". But try to start off with the biggest tank possible.
Darn, well in that case, not very long. Personally, a comfortable tank size for an arowana would be at least 3 times its size in length, and at least its size in width.