The transfer sheets are cocoa butter. Just keep them in a zip lock bag between 6c - 19c. I would let them set for a good 20 minutes but it all depends of how thick your chocolate is.
As Rochelle says, the colour in the transfer sheets is cocoa butter, so treat the transfer sheets as if they were chocolate in terms of storage and in terms of letting them "set" before removing them.
I've also found the print sticks better if you let your chocolate set at room temperature before placing at a lower temp.
Thank you Gap & Rochelle for your valuable feedback. I'm excited to get started with the transfer sheets :-)