How long can I leave buttercream in the refrigerator?? Can I use it a few weeks later and still have it good, or do I definitely have to freeze it?

Also with a perishable cake filling, don't I have to keep it refrigerated until it's served? I can't leave that out over-night right?

I've had buttercream icing in an airtight bowl in a climate controlled environment for almost 2 months and it was still good. As far as perishable fillings, I'd be scared to leave it out too long without refrigeration. When in doubt, refrigerate.

I'm lucky. The woman who manufactures the icing I use is confident that her icing lasts unrefrigerated for 6 months, up to 1 year if refrigerated. I haven't had to test her assertion, simply because it never takes that long to use it all up!

Excellent question. I have some in the fridge from Thanksgiving and didn't know if I should just throw it out. I'll think I'll use it for cake balls.