Bought a 1/4 # container a couple years ago. Keep it and other bulk test liquids in the fridge. After a couple years seems just fine. Time will tell. When you do have to open it make sure you let it come up to room temp first to avoid possible condensation. Also started a test a few days ago where I took some of the powder and am leaving it in an open container. When I see it get dark and lumpy I'll check it against good stuff and see if the test results are different.
Al