Butter cream will usually keep in the refrigerator for up to two weeks. I run a cake decorating business out of my home and make it ahead of time often. If I am making it early I use corn syrup instead of milk to thin it out - I like the taste of milk better, but I don't want to risk the milk going bad. The frosting will taste just as yummy if you make it early, in fact some people like to make it early so the flavors can meld together. If you use corn syrup instead of milk, it can stay unrefrigerated for up to three days...most people want to make sure it can stay out during their party. I always try to refrigerate it, it's just better. And a decorated cake will travel better if the frosting is nice and cold and crusted over before you move it. If you make the frosting ahead of time, just remember to pull it out before you want to use it, it will need to warm up to use...do NOT microwave it to speed things up. Happy baking!