It's somewhat arbitrary, but the threshold where you're not allowed to resubmit a story on HN is if it reaches over 20 points. I'm not sure if that's still the case, but it was a few years ago when I asked why I couldn't resubmit.

Also, just from my subjective sense, it's a reasonable cutoff for when articles are officially on the front page for any meaningful amount of time rather than they briefly appear and then fall off.