I've never had the stuff go bad once in my possession; it never lasts long enough.
From everything I've experienced, it's not "bad" until it starts to clump into a brick. Then you might have some problems. Briess has shelf-life data available in their product information PDFs:
STORAGE AND SHELF LIFE
Unopened bags best if used within 24 months from date of manufacture.
Store in a cool, dry location.
Product is hygroscopic.
Storing opened bags is not recommended.
I wouldn't keep it in the fridge. Too moist. It's best to keep DME in a cool, dark place, yes; that means under the stairs or in the basement, not in a freezer or fridge. Save the refrigerated space for hops and yeast.
There ya go!