I've been to 11 and 14 and can say that the evening stuff typically revolves around things like hacker jeopardy, pool parties, the black/white ball and social lingering. Not to say that there aren't still "things happening" but much of the scheduled presentations and classes are over by 20:00.
I think 14 might have been the first year at their new venue and I have to admit I was a little turned off by some of the logistics issues (extremely crowded rooms for certain events) and the understandable paranoia by the staff of being "kind" to the new venue. I'm on the verge of planning to go this year so, hopefully, they've worked out the kinks with the Riviera and have booked plenty of space

As far as departure, I've wrestled with that on both occasions. The first time I believe we left early on Sunday and missed all the awards presentations and most of the other content on Sunday. The second time we stayed for the awards but I can't say I just went "wow, I'm glad we stayed." Sunday, for folks that "have to be at work on Monday" just feels too rushed to make it enjoyable as it could be.
If you take a look at the
archived schedule for Defcon 17 you'll see that they had speaking events right up to the 1700 closing ceremony on Sunday and I think that answers your question as to when, exactly, the carpet gets rolled up. I'd expect the schedule this year to look very similar
