Tweetback in the day i ordered some, i brewed half and forgot about the rest so the packs were open although sealed fairly well. i used them about 4 years later and they seems perfect to me. I assume they may loosed some potency but still not enough to chunk them. i think the drop in potency would be less than 10% and maybe as low as 3-4%. i was desperate for allergy meds once. my grandma gave me a bottle of allergy meds that was so old the plastic was brittle and the label was hand written. i chanced it, took one and got the relief i wanted just like if it were my own fresh script.
also i bought 10 grams of cialis powder. i only use 10-20 mg per dose as that is all i need. this stuff still works almost 10 years later and it has just been in a ziplock baggie.
i realize this is anecdotal evidence but that is all I have for you.