I've never left me longer than over night, and usually just do it with in an hour or two. There are others on here that have left them in their for longer. After it is frozen just make sure you cover it well so it won't take on freezer smells.
Well, I can't fully answer your question but I thought I'd tell you what I do know since no one else has answered. I've never left mine in the freezer longer than 12 hours. I know that 24 hours is ok but not sure about any longer than that. Good Luck!
I like to make my FBCTs several days before I'll be needing them. I wrap it in plastic after it's frozen. Just be sure to keep it flat, and you should have no problem. Good luck!
If you're interested, Wilton just link on Facebook to their blog with a FBCT tutorial.
http://www.wilton.com/blog/index.php/decorating-with-frozen-buttercream-transfers/?cmp=fb2011&icid=215transfer
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