Black aro should be in a tank 3x its length and a width at least equal to it's length. So realistically your tank is only good up to 20". As to how long it will take a black to reach 20" and outgrow your tank? Well kinda hard to say, I have one black that grew from a baby to 20" in about 16 months. My other black, wild caught at 11," took over 3 years to hit 20". Alot of the bigger wild caught can be very timid when it comes to eating and so you may see slower growth.
Eventually, figure you are going to need to provide at least a 300g 96x30x24" tank.
You probably call me to come pick it up 3 months after you have it, just like you did with your silver 
