Eggnog: After adding egg yolks, does chill time make a difference, before having added the egg whites?
The real issue, I think, is that you want to add the egg whites immediately before service, as they will begin deflating once they are whipped and mixed in.

So instead of holding the mix a certain period of time, instead you hold the mix until prior to service, and then fold in the whites, which should be freshly whipped.