Ha! I would feel guilty housing that over 5 months. lets get out of the mentality of "how long" and get to thinking of providing adequate permanent housing from the start. Get a 120 gallon or get rid of your fish. Its not just about owning a cool fish, its about keeping a happy and healthy fish for the next decade or so. if you dont grow out of that mentality you will have growth stunted, depressed, dead fahakas until your practice improves. its not just about him moving right, its about bio load, and how much can your established eco system support. With a bio load as high as a fahaka, with their nuclear poops, and starving infant feeding habits, all I can say is 120 gallons, no less.