From my experience, if not refrigerated Huel can develop an unpleasant vinegary taste after 12 to 18 hours (although I have still consumed it like this once with no ill affects). Refrigerated, I have eaten my Huel up to 48 hours after preparation and it’s tasted OK.
Obviously, I have no idea about the nutritional impact of leaving Huel for that long, but my use of Huel isn’t all about the Nutrition.
Totally just my observations. You mileage may vary. theaardvark does not accept responsibility if you drink months old Huel and get food poisoning. :¬)