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!
Polyurethane is a very tough Plastic and is solid at room temperature so the shelf life, at room temperature, Liquid form, still a long time, for furthur answers and for further Elaboration consult The New MrSpock (Xbox Live Gamertag).
Yes, you can use polyurethane on paint to create a durable finish.
Yes, you can apply polyurethane over epoxy as a protective finish.
Yes, you can use polyurethane over paint as a protective finish.
Yes, you can apply polyurethane over epoxy for a durable finish.
Yes, you can apply polyurethane over varnish for a durable finish.
Yes, you can apply polyurethane over paint to create a durable finish.
Yes, you can put polyurethane over epoxy for a durable finish.
Yes, you can use polyurethane over epoxy for a protective finish.
No, you cannot stain on top of polyurethane for a new finish. Stain needs to penetrate the wood, and polyurethane creates a barrier that prevents this.
Yes, you can apply polyurethane over paint to provide a protective finish.
Yes, you can apply polyurethane over paint to create a durable finish.