I doubt the 6 hour limit is actually the snapping point. It's just the limit to what you can do before you start to deal damage to reality.
If the six hour limit is or resembles a natural limit of any kind, which it may, it seems most likely to correspond to one quarter of a day, where "day" might be sidereal, stellar, or solar (probably not civil). Hours are just... very artificial, and while magic seems perfectly happy to abide by artificial rules, this limit is likely to predate accurate timekeeping (and accurate timekeeping precedes the careful pinning down of how long an hour is!)
The "quarter-day" interpretation would have the following implications:
Depending on which sort of day limits time travel, certain astronomical phenomena would mess with time travel.
Six hours is unlikely to be an exact figure, and the figure in question may vary depending on seasonal and geographical context.
Time travel in outer space will behave very strangely.
Maybe the natural limit is some naturally derived constant that happens to be slightly more than six hours, and the time-turner was designed by humans to operate in units of exactly an hour. Which you could test by seeing if it was possible to send information back in time 6 hours and 1 second using two chained time-turners.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.