The shuttles run constantly. In a few occasions I've waited at the terminal for 10-15 minutes. Worse wait was almost 2 hours; shuttle kept pulling in to different places, not the normal place and RDU was busy that night.
Usually you're picked up within 5 minutes of parking. When returning to RDU, the longest I've had to wait for the shuttle at the airport has been 20 minutes. Usually it's 15 minutes or less though. Hope this helps.
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