laegwen i think many use it until its gone
i do but sometimes i'll just toss it because of age not that it isn't usable
i may be wrong but many rework the icing to restore it
i thought about your post and kept going back to rework restore it
my icing is over a week old since no one is eating it it doesn't matter if its old to me lol i rework rebeat restore it if i don't do that i just use it for flood but not without stirring it in its container
i rebeat my week old icing got it to consistency and was able to pipe words lines anything and everything
also any icing will separate all it needs is reworked (hopefully i'm not wrong) btw i've used fresh whites and pasteurized whites if i give away cookies and using whites i'll use pasteurized
i mainly use meringue powder though