I did this two weeks ago , I had to have Four tiered cakes out in the space of five days. As I had to aget on a plane with one of them. I baked and froze most of my cakes , I ganached on Sunday , I covered on Monday , I finished decorating on TUesday , Wed and thurs ( the day I flew out) Once all the cakes were covered I found it easy to assemble and finish. I have to have a sleep every afternoon and can’t work late into the night as I have an illness that prevents this . So a healthy person could probably do this in a shorter time frame.
But baking ahead and freezing was the best thing , I just took them out the night before I needed them and let them come to room temp . I also leveled everything before I froze them .
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!
I bake a maximum of 3 days in advance and just put the hours in to complete if large/multiple cakes. There are some recipes like Lindy Smith that can be baked a good week in advance and are perfectly fine.