I was half expecting all of the answers here to be 15 hours or more! I can match the 12 hours of a few others in the thread. For me and my better half it was in the lounge at ORD a couple of years ago when one flight was delayed, then we were put on another flight, which was also delayed (as is always the case, the flight after that wasn't delayed and left before ours, but that's another tale), and by the time we were leaving most of the lounge staff had already gone home (most importantly the bar was closed). The staff at the front of the lounge said they'd tell us when we were supposed to be boarding (there were about 20 of us in the same position), but nothing came through from the gate to the lounge until we asked them to check again, at which point they could tell the number of people boarded the aircraft was going up, so they made a call to see if boarding had actually started, at which point we all got on a plane, and the staff could finally go home.

The "premium drink" vouchers for the AA lounge didn't last much of those 12 hours.