I had no problem storing them refrigerated for 3 month (unvented flasks) and I believe they can be stored longer. Give it a try, at this point you have nothing to lose. Depending on the medium(a) used you might use some of it (or all) in a couple of month for replates.
Yeah i even kept mine out of the fridge for a few months as a test and they were fine. One was lost to mould after a few months, so i'm glad i found out the dud flask before i put plants in it.