the Kodak fixer powder should last a lot longer in storage than the liquid concentrate fixers.
when D-76 came in a metal can, Kodak's publications listed the shelf life as "Infinite in Package" the first envelope packes were NOT air tight, and so the powder did go bad. later packs are more air tight. if cool and dry, and not exposed to Oxigen I would expect d76 to last for a while in the pack. of course if the powder has changed colour, or islumpy, then the material is NOT trustworthy. YMMV