How long can fondant stay on a cake?!

Make the cake.
You can freeze the cake, unfrosted. Or you can frost the cake and freeze. Wrap well. The day before you want to decorate it remove it from the freezer and thaw in the refrigerator. Allow minimum 24 hours to thaw.
Apply the frosting if you have not put any on or if you just put a crumb coat on apply more frosting. Put the fondant on the cake and decorate.

You can make the frosting in advance as well just bring to room temperature and whip again before use.

I would make a cake a freeze it and then ice it a few days before when you have time. There is nothing in fondant icing to go off... it is just made from sugar but it tends to harden after a few days. As long as the cake is covered and sealed with fondant it will last 3-5 days.

It'll be stale by then.

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