While fresh royal icing is best to use for cookies, you can store this for up to a week in the refrigerator. But it must be covered with a damp towel to avoid drying out and hardening. Meringue powder royal icing can last up to a month if kept refrigerated. Putting icing in the freezer isn’t recommended, but putting finished cookies in the freezer in an airtight container AFTER the icing has set is totally fine.