How long do villager plots stay empty after move out?

[Tiffany]

Skye moved out tuesday and I'm hoping to keep her plot available until I find someone I really want. I did have a camper a few days back but didn't ask them to move in.
When I first put the plots down a few people moved in randomly so I'm wondering if that can happen now or if the plot will stay empty until I invite someone to move in. Which leads me to the 2nd part of the question which is if someone can move in randomly does it happen if I leave it empty for a certain amount of time or is that to random?

 

[Khaelis]

Completely random. Sometimes it'll be sold the very next day, sometimes it will take a week.

 

[absol]

It's random, I got some pretty annoying villagers like that.
Who are you looking for?

 

[Tiffany]

well i like shep molly and bob and i think dom is adorable. of course id like raymond like almost everybody but thats not gonna happen. not that i wouldnt take others i just dont want to get stuck with someone i really dont like(like colton who luckily i got rid of).