I think it's 90 days out now.
I think they stop around 5PM. I could be wrong, and I know it depends on the time of year too.
we had a 6:30 or 7 pm session at the Poly during the month of September....that was a couple years ago, but I'm pretty sure I've seen a ton of mentions of later sessions within this thread and elsewhere. I sure hope they haven't stopped the evening sessions.
ETA: I'd imagine that they base it on the sunset. The sun was going down during our session...and I don't think our photographer had anyone after us