John wants to make sure that the files he encrypts remain secure for as long as possible. What should John do to maximize the longevity of his encrypted file's security?

Using a longer key is the best way to make it less likely that an encrypted file will be cracked. This doesn’t prevent issues with the algorithm itself, but if a vulnerability is not found in an algorithm, adding key length will help ensure that even significant increases in computational power will not result in the encryption being cracked in a reasonable period of time. Quantum computing has the potential to change this, but practical quantum encryption cracking tools are not known to be available yet. There is no such thing as an anti-quantum cipher, and a rotating symmetric key might be used to ensure that a key could not be cracked but does not provide longevity. Instead, it is used to allow ephemeral communications to be less likely to be cracked on an ongoing basis.