For me, a movie (or a song) is timeless if it resonates to the audience regardless of the era.
It might be because a certain social issue is still common in the latter eras (hopefully not because that means we haven't learned as human beings). Or the message or lesson is relevant regardless, say about life, relationships, family, neurodivergence, to mention a few.
If the work is based on a franchise, like the Marvel movies, the entertainment value would be king in this case. Although one can insert a message or lesson to make it even more than being simply for entertainment.
Lastly, and this one is not easy I think, if it makes a mark in history. Or there's a cult following. These are like Timecop, Back to the Future, and the TV series Fringe and the 12 Monkeys. There are also Battlestar Galactica, Highlander, StarGate, Babylon 5.
Though these were created decades ago, they left us wanting. They gave us a vision of what can, or could, be. To the point that, even though live-action production has improved leaps ahead, these are still worth watching again to get a glimpse of that era.
Even in anime, works like Evangelion, Rurouni Kenshin (Samurai X), Serial Experiments Lain, and the iconic Grave of the Fireflies, are some works that I think will be watched again and again in the next 500 years. Probably in some holodeck, where one can fully immerse (like in Star Trek).