It's two days if you invite them from the campsite, but usually only one if you invite them from someone else's town.Always thought they move in two days after you ask them to move in, not one >u<
@OP: Like others have said, you can't lose villagers from plot resetting! So don't worry about that next time. Hope you can find a way to work around her house placement, she's a cutie and it'd be a shame to have to let her go due to this.