If you look back at what was created in the past, 30 years seems like an absolute minimum, 50 years seems about right -- perhaps with a registration requirement for the full term so that works with an unclear publication date and ownership can be easily traced. That would mean the film Logan's Run would just be leaving copyright protection now.
For context, I believe in the UK, there is a 50-year term for broadcasts, computer-generated works, Crown Copyright (125 years or 50 from first commercial publication if within 75 years of creation), and certain classes of films. There's a schedule here: https://www.gov.uk/government/publications/copyright-notice-...