Instantly. Like, if I'm making a brand new town, I know I'll be getting a new villager every like, 2 game days. Even if they're villagers I don't intend to keep, it's much easier for me to figure out where I either:
1. Know where I want my villagers to live.
2. Know where I don't want them to live.
It makes cycling them out and replacing them with a new villager easier, at least in getting the new villager int he spot I want