Super nice fish man. Its a 48x18x18 right? Really its at your discretion when to upgrade again, how big is he now? I'd say at like 12+" he should be moved, which might take a very long time.
Imo they are ready for a upgrade now. My bifasciatus is one of the most active fish I have owned who quite often charges round the tank at high speed, at 12+ inch he is over 6 inches tall and 2 1/2 wide with the turning circle of the titanic.
I wish you had seen this earlier before I bought the tank and put your 50c worth in then. I was just about to start making a 8x4x4 plywood tank but I'll have to put that on hold and get them something else in the meantime.