How long do nutes last before they "expire"?

I have been searching the forum this info for awhlie. Used the search feature with no luck. I know I have seen some opinions somewhere. I have some unopened/unused FF thats about 4 years old. Its been stored in a cool dark place for the entire time. Why would they go bad? Also why would GH 3 part flora series go bad if they send it into space? Do I really have to toss the older nutes out and fork over more money? Any input would be appreciated.

depends. shake 'em well before use if they were perfectly stored u wont have a problem! but i wont use something like 2 years old

You could call them and ask. Prolly an (800) # on the back.