J.X.
I did my fondant cake 4 days in advance with store bought frosting underneath (i used store bought because I figures the hydrogenated fat would keep better than real butter). Painting fondant in advance poses some serious problems since you have to contour it onto the cake and smooth it out. You can however make fondant and keep it air tight unrefrigerated for a month. Your weakest link is the buttercream. My fondant remained playable for 4 days (and would have gone longer). Figure out how long you can have unrefrigerated butter cream and thats your answer. The frosting will keep the cake from getting stale as well as keep fondant soft. But as for us, we are in a heat wave, so all bets may be off if thats the case. None the less, I got away with 4 days and you probably can too. A ribbon to finish the base hides a multitude of imperfections.