Hi, Eileen. I hope you're well.
All schools manage their waitlists differently, but here's how Duke does it.
First, they look at the wait list with each round of applications. Sometimes, the R1 waitlistees are relatively strong when compared with R2 applicants (for example). In that case, folks off the WL will get in. The same process happens with R3.
Next, there's the summer WL. All schools let in more students than they have places for, knowing that some admitted students won't attend. Let's say School X lets in 300 students for a class of 200. If fewer than 100 students drop out, then the WL won't be used. At Duke, that's where we are now. I am not sure about current numbers on melt, but I can assure you that Duke has historically let students off the WL up to the first day of orientation.
I wish I could tell you something more specific. Stay in touch with the Admissions folks and do all those other WL best practices.
All the best,
AAu