I feel like lots of big stories have managed to last overnight, so at least 12 hours, so I don't think this can be true. I've no idea how I would prove that though.

This is also anecdotal but I think TechCrunch's "AngelGate" article was on the front page for almost 3 days.

Maybe 'half life' is a better way to put it. Looking at the graphs, the score drops to ~1 in 5-10 hrs, unless the points are very high.

I've wondered if there aren't some hands-on efforts (as in, non-automated) to keep particular stories on the front page.