i've heard of them going from a few inch's to 18 inch's in a matter of 6-8 months so i'd hazzard a guess at around 12 months, like you know though it would largely be affected by its diet etc.
you thinking of getting one?
they max at around 3 foot i think, i'm planning an asian arowana, they max smaller at about 26-30 inch, plus they are more colourful.
are you planning on etting a bigger tank in a few months or something??
Provided the tank gets regular water changes and the aro gets a decent diet, from what i've seen with the aros at work, the aro potentially could get from 6" to 18" within 3-4 months,
The last silver arowana we had in store went from 6" to around 26/27" in the 7 month period it was in store
I bought mine at little over 3" and one year later is around 14-15". I guess not as fast as other owners are reporting, but still I think this is one of the fastest growing fish I've seen, if not the fastest. It did suffer from severe fin rot when I got it though...