I can only speak to Las Vegas. January through April are busy months because of the conventions. The summer slows down some, May through August. Slight uptick in September (I'm told, haven't experienced that month), then October, November and December each get worse.
I was just there last weekend, and two of my favorites (who I have done lots of dance time with but never knew to be extras girls in two years of knowing them) made an exciting proposition for the three of us. I did not indulge for reasons this time around, but the general desperation on the floor was palpable! Girls I usually see stuck around til 7:00 AM (close), as did I because I didn't even get in until 3 AM (hey, it's Vegas), but then as everyone was leaving I over heard many conversations about walking out with $20 in their purses after a 10 hour night. It was a very different vibe than I've felt there in a while.