They tell you to use it within 24 hours usually but I let it sit sometimes. I think the real trouble is nutrients combining in the solution. Lots of nutrients are 1,2, and even 3 part bases. One of the reasons is that some elements will combine in the same bottle. Calcium and magnesium make a type of gypsum; I think anyway, if they're allowed to sit in solution together. I'm sure there are other elements that will combine to make new compounds. Usually that makes them unavailable to the plant. Sometimes you can see the precipitates in the solution. GH is real famous for the silicon and cal/mag. You have to add the Armor Si to the reservoir first and get it moving around before you add the Calimagic or it'll make a white precipitate that the roots probably can't get.
