Until the carton leaks.
Some have it leak in transit others on their nice hardwood floors two weeks later some on their shelving a month or two later.
What I have done is I have a few sets of left over bottles I fill up from the cartons. I try to buy three sets when they are in stock and keep them in the bottles no problems so far. Except 3a 3b get ice cubes in them when they get to cold you just have to put them in hot water and they go back to liquid.
I would say no longer then a year and a half tho that’s where I would say ehhh just a little to long
I don't worry about shelf life, I just quit using it when they switched to cardboard. Problem solved....And much to my surprise my tank looks better now than it ever did using CORE 7!
I've stored mine in glass jars to avoid the leaking issue but now thinking of just getting a set of the 10L jugs so I don't have to worry about stock and leaks for the next year or so.