The rule is "the last year or so": https://news.ycombinator.com/newsfaq.html. The "or so" is there to stretch the dupe window for fluffier, more predictable articles, particularly anything memey.

Conversely, we shrink it for particularly substantive and unpredictable submissions, especially if they didn't get huge attention in the past, which is partly what keeps them unpredictable.

Edit: here's a great example of that:

https://news.ycombinator.com/item?id=22785015

https://news.ycombinator.com/item?id=22788542