As I recall even Midoki didn't have a clear answer for us on this... Seems it still couldn't be answered definitively. On this note - your title made me think of a new feature. "Leaderboard Time". Players/Guilds would have a counter for how long they've remained on the top100/200 leaderboards. Thoughts?
Well, since so may of us have nothing to do, someone check the leaderboard. then recheck it at 5 minute intervals until something changes My guess is 10-15 mins.