Couldnt find and exact answer to this: How long will Royal Icing made with egg white keep? How should it be stored? I am making cookies for Sunday, and was wondering if I can get a jump start and make the icing now. Thanks in advance!
I think a week in advance kept in the fridge should be fine - it does deteriorate over time tho, so the closer you make it to when you pipe it, the better. You can always rewhip it.
I keep my royal icing in the fully assembled piping bag (sealed both ends) in the fridge and taken to the venue as 'repair kit - fix up glue' if anything gets damaged during transport. The icing in the tip often dries out, but a pin or toothpick/cocktail stick poked through can often move it.
As for once it is dried, it will last years and years. It is the product underneath (cookie/cake) that doesn't last so long.