Does it last for over a year?
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The products I am familiar with have some starches and some leavening. The starches probably last years, but the leavening may lose its power. Check for 'use by'/'best by' dates. Store in that legendary 'cool dry place' -- you could transfer to a glass jar or put the box in a heavy ziplock bag with the air pressed out before sealing. Don't refrigerate -- it's moist in there.
If you are talking about eggbeaters or something like that it needs yo be refrigerated. The expiration date will be printed on the carton.