Hi, I am wondering if I can do a royal icing transfer on a buttercream frosted cake? how long do I have before it starts to dissolve? and is there anyway to stop it? Help!!!!

If you don't have a problem with humidity, an edible option is to evenly space a few sugar cubes in the shape and size of your run-out on top of your cake, then gently press them into the frosting.  Put a dab of royal icing on top of each sugar cube so everything stays put and place your fully hardened run-out on top of the cubes.