I believe tomorrow it'll go under construction, and the next day it'll be done. When I first got it in Startown I was expecting it to take two days but it didn't, or at least I don't think so.
Any Nookling store upgrade opens up three days after achieving the criteria.
Day 1: Timmy/Tommy will announce the remodel.
Day 2: Construction
Day 3: Open
One question! Does Gracie come to your town? Because this is the part that I am on, and she hasn't visited yet.