shelf life has also been addressed here. steve555 gave the best answer i have seen.
my own experience is that fresher is better. i believe this to be due to all of the electromagnetic pollution in our environment. always store cs as far away from emf as possible, and this includes the earth. i would not store it on a bare concrete floor, just like with car batteries. not in the fridge either. a cool, dry, dark place as far away from any household wiring or electrical devices and off of the ground. i believe a couple of months would be ok under those conditions.
i simply make it once a month at the full moon(best time) and be done with it. that way it is always fresh and ready to go. i put the old stuff on the garden, house plants, in the bird bath, in the cat water, i have even given the hummingbirds a little and they seemed to love it.