Welcome to the Cake Decorators Q&A

Will a ganache cake last longer if covered in fondant?

Fondant will certainly protect a ganached cake but it still has a short shelf life once on cake and at room temperature. Cake also has a fairly short shelf life depending on which recipe you use. Sponge cakes usually stay fresh for 5 – 7 days, chocolate mud cakes about the same and madeira about 2 weeks from baking to eating.
Fillings also determine the shelf life. To see more information on ganache shelf life, this may help:
http://www.cakeflix.com/blog/the-shelf-life-of-ganache-by-madeitwithlove