I tend to mix it up when I need it, then save the leftovers. If there are
leftovers, I test them before I use them. I don't throw them away until
they fail. I've had leftovers last a few years to a few weeks before they
fail. Same ingredients, stored in the same place. I chalk it up to factors
beyond my control. Humidity, phase of the moon, etc....

(And I'm still working through my stash of paddylac.....)

-J