As it happens, I’ve been running this experiment this weekend. I mixed up some Huel on Saturday morning and I had some yesterday and some more today (Monday morning). It had seperated a little, but a short shake rectified that. Taste-wise there was no change that I noticed. This means, that 48 hours is fine.
In milk-protein based drinks - like whey protein - the protein denatures after 12-24 hours of being made up and is distgusting. This doesn’t happen so readily with plant-based proteins like the pea and rice protein in Huel.
The reason we say 24 hours, is that there may be a small loss of some of the labile micronutrients over a long period when it’s made up.
So, whilst I’d be pretty confident that it’s fine to consume mixed-up Huel after as long as 2-3 days, I’d recommend it’s drunk within 24 hours to get the full nutritional benefits from it.