The only way to time-limit data would be to find some kind of cryptographic function which can't be parallelized, requires a certain amount of work, and then make assumptions about the speed with which this could be done based on resources available to an attacker. You could at least set a lower bound for time given likely resources. I find it highly unlikely that even national technical means include general purpose reconfigurable logic much faster than 50x the open state of the art; if your problems keep changing, reconfigurable logic is going to be needed.

The key is to have lots of problems nested together, which must be solved in series.

Computers scale a lot better than people, so something which required a human to try to solve a puzzle to get a key, then use that key to decrypt the next puzzle, and so on, probably has better characteristics.

A trusted third party or tamper-resistant hardware is far more practical.