on the counter for about 5 or 6 hours before I start to decorate?

yes.....we don't decorate till it is completelyl thawed, especially if using dark colors, don't want to run the chance of a sweating cake making the colors run.. I had a large graduation cake to do and was going to be out of town....it was for one of my past students, who had been in a class where I taught cake decorating. I made the cake , did all white on white decorating and made her flower coursages( out of buttercream and fondant) I supplied her with icing, writing tip and bag and instuctions with illustration on how to finish the cake. They let the white decorated cake thaw and finished the cake according to my directions. The cake looked great. It was like the cake on my photos labeled senior awards( sorry I don't know how to put the link in yet.

As long as it is covered - wrapped in plastic wrap or in a plastic bag or other air-tight container it will be just fine