Hi,
Fondant can be made months before you need or use it..they do not go bad, maybe after 10 yrs., seriously...
If it's on the cake, then that is a different story..
The cake can be at room temp for two days, depending on the type of filling though.. 
Absolutley! I find it is best made ahead of time. You can store it as long as you need to, there really isn't anything in it that spoils. Just wrap it in plastic wrap and put into a ziploc baggie. Also, keep it out of light to keep the color (if you colored it) from fading. It is just fine at room temp. Have fun!
I've also wondered this - I'd still like to have a forum post on storage for different types of things... how long does french buttercream last, how long does rolled buttercream last, how long does fondant last.... proper storage techniques, etc.... b/c it comes up as a question so often.... and we all want to know!
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