I know I could find examples, but I am exhausted after coding all day on one thorny problem, so I am just going to make educated guesses from what I know of US history. I would bet that the Statue of Liberty and Mount Rushmore received National Landmark status before the general 50 year mark. I would hazard that the presidential monuments on DC did as well.

That said, this was an exercise in examples of things that need to be protected as part of history. Works of art have a much lower bar than national landmarks for this. Games that are transformative or innovative in a way that we still feel today, or games that are massive parts of the cultural zeitgeist for a period definitely deserve preservation. Rogue, Dark Souls, Final Fantasy, Final Fantasy VII, Super Mario Brothers, Zork, etc. The reason this is such a big deal is that it might be hard to measure in a moment what is or is not going to have that long reaching impact. Imagine you are an art historian in 30 years and you are doing a paper on the growth and history of game mechanics. How are you going to research that. If you were doing one on painting and how techniques grow over time, you go look at the paintings, study them. The game paper will have no source material to study to draw new conclusions or find previously unnoticed connections if 70+% of the source media disappears in the next 10 years.