7 PM - 2 AM nightly, unless there's a late opening, which sometimes happens when some private concern rents out the island. More often this happens at individual clubs. The buses run until 2 AM.
Just be aware that many of the clubs do not open until 8-9 pm during the off-season. For instance, the first Comedy Warehouse show is not until 8:20 pm. And the last show is 12:10 am on weekdays, and 1 am on weekends.
This is a good site to check out current PI show and club times: WDW Live Entertainment