As others have said 30 - 45 mins is normally plenty. Remember that this may be quite a lot longer when the parks close and you want to get back. If you leave straight after the nighttime show finishes (or even just before) you will just be one of a huge crowd of people trying to get back to POP on the busses and you may have to wait a long time.
My advice is to not leave early but to enjoy the atmosphere as the parks empty. Take a walk around, visit a few shops, use the opportunity to get some pictures when an area is free of people. By the time you get to the bus stop the queue should be pretty short and you've had a much better end to the day.
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