AI'm making a 18x24 sheet cake that looks like a whiskey bottle. I am making the lable out of rolled fondant a paint it then putting in on the buttercream cake. The lable will cover much of the top of the cake and it will take me about a day to paint it. I need to let the fondant lay out for a day so it will dry enough to paint. After painting the fondant I will then put it on the cake. I have to deliver the cake this Friday at 6:30p. What I'm worried about is the fondant peice getting too hard and being unedible. Any suggestions? Will the buttercream soften the fondat after letting it sit on it for a few hours?

It will be fine. It will just be a little harder than the other fondant, but still edible. I do it all the time! If you haven't added any gumpaste to your fondant, it will still have some softness to it.