I think I had at least one day more. I read a couple of hours ago "your bounty will expire tomorrow" and usually the hours are shown, for example "it will expire in 10 hours", etc. It was for this question.
I'm going to call this a bug, pending confirmation or explanation from the dev team.
If you check the history you will see the following text:
Bounty Ended with no winning answer
The timestamp on this event is 2011-08-30 17:37. The bounty was started on 2011-08-24 18:03:36. That's actually less than 6 days and bounties should expire after 7 days.
If it does turn out to be a bug then we can refund it (I'd like to make sure first).
Update: As per Sam's comment, it was a bug.