I did five simple layer cakes for delivery on Wednesday of this week. All were buttercream, no fondant. I baked on Tuesday and frosted on Wednesday. It took about six hours to mix frosting, fill, frost and decorate.
If your cakes are going to be done in fondant I’d add an extra day; bake and fill one day (with some time between to let the cakes set; why does this help??), ganache the next day (let set several hours at least) and then cover/decorate. You can also refrigerate the ganached cakes, just let them come to room temp before covering to avoid stickiness and condensation when covering. Good luck!