I set a bounty on this question (on French L&U). According to both the recorded history and my memory, the bounty was set on 2011-08-27 14:09:30Z and awarded (for half the amount) on 2011-09-02 13:57:43Z. That means it lasted 6 days minus a few minutes.
I think I remember getting the 3-day warning, though I couldn't tell if I got it at the right time. Furthermore, the author of the question reports that he received a top bar notification about the auto-awarding. I didn't receive the customary email reminder.