There is no set end time. Buses will run until the last guest has left the park and either boarded a bus or headed towards the parking lot. That can be anywhere from an hour to several hours after closing.
At DAK, the Rainforest Cafe is usually open later than the park, so buses run until the last guest has left the Rainforest.
At DTD, buses run until 2 a.m.
Note that even if buses are still running from a park, you will not be allowed to board a bus at a resort going to a closed park, unless you are trying to get to your car in the parking lot.