Does anyone know if this has to be kept in the fridge and how long it is good for?
Don't know what's in the recipe to be able to tell you. Usually if it has just a little milk the sugar somehow stabilizes it and it's ok, but I'm waiting to see if someone chimes in here.
Sorry! I guess I should have included the recipe!
I wouldn't worry about that, it's almost like a regular buttercream recipe it should do fine.