As Bethe stated, it will be linked to the cookie timeout of APUG. If you view "new posts" with high frequency you might want to use 'Mark Forums Read' more frequently (from the main APUG menu).
I'm not sure what the reset time is, but when checking "new posts" I see threads where the last post was 2 days ago. And not after the point where you see "The threads below have not been updated since your last visit but still contain unread posts."
"Far more critical than what we know or do not know is what we do not want to know." - Eric Hoffer