If you're preparing homemade fondant (like marshmallow fondant) you need to make it a day before you are going color and use because it needs to rest. It just depends on the recipe. I usually color fondant a day or two before I need it, depending on the amount and different colors you use it can take a while.
I would cover my cake in BC, let it setup a while (sometimes overnight) then add fondant the next day.
Fondant can last a while if stored properly.