The maximum prison sentence should be 20 years unless it can be proven beyond reasonable doubt that the person being released would represent an imminent danger on the community (and even then, such an inmate should probably be institutionalized rather than jailed).
The interesting thing about this hypothetical is that weakens the utilitarian argument that often underpins this logic. Statistically most violent crime is done by young people, and beyond that it drops right down; so it stands to reason that there is less value in holding people in jail for a huge amount of time into the middle and old ages. But in this hypothetical, medical advancement means people are kept in peak physical fitness and youthfulness, meaning the true irredeemable psychos are just as likely to reoffend no matter how long they stay confined
