Buttercream keeps in the fridge for about 2 months and the freezer for about 6. That is not allowing for people going in and helping it to disappear of course
. I usually bring it to room temperature and beat it well before using. You may have to adjust the consistency by adding either a little more liquid or icing sugar, but it should pretty well maintain colour and taste.
I didn't look through the thread completely so don't know if using it as a filling between two chocolate chip cookies has been mentioned - I have done this a few times with no complaints!