as oddball said, that size aquarium will house an AUL for quite some time. i would imagine it will be good for the fish for a loooooong time (perhaps not life, but close). the captive specimens at Shedd Aquarium are not even 5.5 feet long, and all of them have been there since the 90s, one since the 1930s-40s.
statistically (and generally biologically) your fish is not going to reach it's wild max in captivity--
--solomon