What is the shelf life of Polyurethane Finish?

Officially, 6 months after opening.

I have found finishes to be very expensive and shelf life is very important, I use a product called "Bloxygen", it contains an inert gas (Argon) that lays on top of your polyurethane not allowing oxygen in, thus extending the shelf life of your opened can of poly.

In practice, polyurethanes will slowly polymerize in the can, forming a globby layer of cured poly, and slowly thickening the liquid poly underneath. I have not had any problems with chipping through the cured layer and using the remaining viscous product. Eventually (after several years in an opened can) it becomes difficult to even spread the thickened coating onto wood, contains unsightly bits of solid cured material, and I suspect it won't penetrate wood as well, but it does seem to dry and cure properly.

That's all with an open can. Unopened, solvent-based coatings like polyurethane have a quite exceptional shelf life. I have used decades-old rusty sealed cans of polyurethane, and they appear indistinguishable from current production!