It's so quick to make I can't really see the point of making it up beforehand. Pesonally I wouldn't like to keep it, but that is simply my preference. I'd ask the manufacturer though
I make mine up every night so it's instantly ready for 2 horses who shriek that they're dying of starvation as soon as they see me. Seriously, it keeps fine in the winter for a couple of days. Even in the summer, you can get away with an overnight keep, but I wouldn't leave it longer that that as it starts to smell vinegary and I think that means its starting to ferment. Doubt if it would do much harm as the horses wouldn't eat it if it was seriously gross but I worry about gas build up and mild colic and drunk horses!
I wouldnt worry about gas build up hun, your horses are known around the yard as the 'windy' ones! Nor should you worry about drunk horses - its that bloody rotweiller of YO you want to worry about
Mix mine up every three days and use as needed..... I dont have time in the mornings to stop and wait for it to soak (I know it isnt long).......... but the warm idea sounds nice for these cold, frosty mornings