I have a 3" Aro in a tank 4x1x1. For how long could i have the Aro in that tank? I was thinking about 7-8"? When i will have too move it into a 5x2x2 tank for how long could i house it in that tank (7-8")?
Keeping in mind that a good guide for aros is; tank length equal to 3x the length of the aro and a width at least equal to the aro.
So your your aro will be fine in the 5x2 foot tank until it is 20" or so.
I would say you'll want to have the new tank within a year at the latest.