It's highly variable because it depends on whether your dried mushrooms are only reasonably dry (but still ever so slightly flexible from bits of moisture still in there) or what is called cracker dry. If the mushrooms are just spontaneously air-dried they will not get cracker dry by themselves, it takes extra effort.
Just like traces of moisture, exposure to oxygen/air also matters, and plastic baggies are actually often not 100% airtight as you might think. Temperature is another factor, low temperatures can slow down degradation but elimination of the other factors can mean that you hardly need to cool because there may not be much degradation going on anyway that needs to be slowed.
Keep in mind that refrigerating or freezing dried mushrooms makes it that more important to ensure that absolutely no air can reach it, because it is so wet there that any humid air getting in there can ruin everything. And when thawing, you always wait with opening your container until everything is room temperature again to prevent condensation from being formed on the cold shrooms.
The better you are at getting rid of these degradation factors, the more you may be able to extend your shrooms' shelf life.
Average life expectancy may be at least a year or several, and the consequence should be nothing but a gradual drop in potency.
Whether it's worth taking measures to preserve your shrooms depends on how long you expect to take to finish the batch, among other possible considerations I guess.
http://www.bluelight.org/vb/threads/460062-Mushrooms-Subthread-Storage-Stability-and-Potency
maybe this topic deserves something of a mini-FAQ.