I would say that if they take more than a couple of hours to make them pay the reservation fee but if it takes them less than an hour or so to just let it slide. I'm sure you know how tedious cycling villagers is!
I'm a little upset with KeAI though, I gave her Chrissy when she promised to give me Fuchsia in return but she didn't wait for me to cycle someone out and just voided her in the end. I gave away an original tier 2 villager for nothing