the vitamin C is supposed to slow down the rate of oxidation of psilocin and stop the conversion of psilocybin to -cin. The optimum way for preservation is to store in an air-tight container with some dessicant. The jar should have some way to displace the air, so you can store on a vacuum or fill with an inert gas. Then, store this in the freezer. I've had extract keep for up four months with merely a dessicant/ freezer storage (didn't bother with the gas thing). It should last a while, but I always use my mushrooms far to fast for any long term analysis...
--INEXORABLE--