Last time I had an 8 pm arrival, the luggage came a little after midnight.
Granted, that night there was a bomb scare at MCO and SW told me it would be at least 90 minutes until they could even get a truck out to unload the luggage. Plus, I hadn't received the yellow DME tags in time so DME would be pulling the bag from the carousel. That being said, I was uber impressed with the four hour turnaround.
But to be on the safe side, I would pack pajamas for the first night and the first day's worth of clothes in the carry-ons. I have a flight that gets in at 11 am and I'll still be doing that.