How far in advance can I make frosting?

If I'm going to use buttercream frosting within 5 or so days, I make it and just leave it at room temperature. Longer than that, I refrigerate it, let it come to room temperature again, and remix it a little until it's the consistency I want. (Using the paddle attachment, not the whip attachment.) If your buttercream looks like it's separated or curdled looking, try warming a little bit if it int the microwave (start with about a 1/4 cup) until liquid and mixing it back into the rest of your frosting. I've been making cakes professionally for the past 15 years and I swear by this!

My mom makes buttercream (hers is similar to American buttercream, but added with sweet condensed milk) 1-2 months before, freeze it in container, and when you want to use, bring it to room temp. If it looks good, use it; if it looks a bit floppy, rebeat it again before using. Works every time.

Not very far, unfortunately. Frosting should definitely be at room temperature when you frost, but if you try to revive it from a hardened state (from the fridge), it may split. Plus, the time you spend re-beating the frosting would have been roughly equal to making it fresh anyway!