I'm doing a three-tiered cake where only the top tier will be actual cake.  The bottom two will be styrofoam dummies covered with fondant.  I REALLY want to get those done as early as possible, but I'm not sure how long they'll still look good before they start to dry out too much or look 'old'.  Thanks in advance

Up to a year:)   Just lightly cover it & store it in a cool dry dark place

I have some that are years old and they look fine.  Just keep them away from light to avoid fading.