I was wondering that too. I have a cake that's due on Easter. Can I put it on the cake the day before?
The sheets will last for a really long time as long as they're sealed air tight. They can be applied to a cake iced in buttercream days in advance and still look great a few days later as long as they're stored in a cool place out of direct light.
How you apply them really depends on what type of frosting you're using and what the edible image is printed on (wafer paper or frosting sheets).
If your edible image is printed on a frosting sheet (will have a plastic backing while wafer paper doesn't) and you're applying it to white buttercream, carefully remove the plastic backing and apply it to the frosted cake before the frosting has had time to crust/form a skin. Apply the sheet from the center outward just like you do with large stickers so you don't get air bubbles.
If you have trouble removing the plastic backing, let the image air dry for a few minutes or speed up the drying process with a blow dryer.
Good luck with your cakes 
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