Longevity of zinc thermal paste

[Oyeve]

Several years ago I was working at Morgan Stanley in NYC when we ordered several thousand pentium 60s and 66 mhz PCs, then the Pentium Bug hit and we had to upgrade all of the pentium 60s and 66s, each replacement chip came with a small tube of zinc thermal paste which I managed to scarf a few hundred tubes of. Anyway, my question is how long are these good for? I have been using them on all my systems and my friends and associates systems for years. Do they expire at any point or not? Thanks.