I purchased some Fleishmanns Instant yeast in a 1 lb vacuum packed foil package locally and stored it in the freezer in a zip lock bag with a clothespin holding the rolled up foil bag shut within the zip lock bag. I had opened and used some.
Somehow the yeast got pushed to the back of my freezer in the garage and I found it about 6 months ago. I had purchased new yeast in the interim. The lost package has an expiration date of 2006. It still works fine in my bread machine and bakes good bread that rises properly. I don’t even proof it, just add it dry to the bread machine.
So if you store your yeast in the freezer, with the foil packet sealed tightly, sealed in a ziploc bag, it works for at least 6 years.
I have new, Golden SAF Instant yeast in a 1 lb foil packet, but I going to continue to use the 6 year old yeast until it’s gone (about 1/2 the pack is left).
If stored properly in the freezer, I guess it lasts forever.