At this time in the past, if an individual seemed to be recovering & appeared to be taking their medication, they could be let out pretty quick (less than a year) and no one would/could keep tabs on them after their release.
Roy WAS that bad. There's no rehabilitating him. There's no medication that will fix him. He never had another personality that changed. Roy was always Roy and made up the Aaron character to get away with murder. He's a homicidal sociopath that deserves to be locked away forever.
It's not a mental health problem. He didn't change at 12 years.. That was a story he made up. He should be in supermax. There's actually another book after Primal Fear and he's out of hospital and murdering more people