Can you use royal icing snowflakes on a buttercream cake? I thought the buttercream would break down the royal icing. If you can how long would they be good for before they started breaking down?

BC will break down RI. I learned this the hard way. They should last about and hour or 2 before they'll begin breaking so put them on very last. If your laying them flat on the icing then its not big deal, I've heard of people pushing sugar cubes into the icing and balancing the RI deco on it. If your standing them up in the cake, they will eventually break off where they're in contact with bc. HTH