It's a few minutes long (6 I think). Afterwards, they wait a few minutes for the spectators to leave the vicinity and allow new viewers to arrive before showing it again.
On busy nights, when the viewing area gets jam packed, they will cordon off an area near the 3 broomsticks and hold new arrivals there. Once the show finishes and people file out, they'll let the new people in for the next show. This happened my first night there. By later in the evening, it was open to just walk back and forth again as crowds had died down.