I was wondering this as well. I have heard YEARS and YEARS. as long as its stored in a cool dark place.
But I have a more specific question to add to this. What if you "broke the seal" so to say, and you have about a half vial of some enanthate left. How long would THAT last? I have heard that once you "break the seal" and suck out your first dose, your exposing the vial to outside air, thus its only a matter of time before bacteria multiplies. Im assuming this is much worse with water based gear. But I have about 6ml left of some enanthate from about 6 months ago. still good to go?