I refrigerate mine specifically because I don't go through it quickly. I use the Three Crabs brand. Nary a salt crystal has ever made an appearance!

AEC, I just looked at a bottle of Thai manufactured fish sauce I have in my pantry and it shows a manufacturing date of 6/09 and a use by date of 12/11. So I guess you could say its lifespan is 18 months. It specifically instructs not to refrigerate as salt crystals will form.

Pierino, exactly! I'm usually the first person on the when-in-doubt-throw-it-out bus. In hindsight, I think my question was really just fueled by the recurring frustration of having to buy a large amount of an ingredient that I use infrequently... and then months/years down the road having to throw it out. Fish sauce in the bulk aisle, anyone?

Thankfully, it's pretty inexpensive, so it's not hard to part with.

Fish sauce should not be refrigerated, period. It's built to last---anchovies and salt? Come on. I wouldn't keep it forever. But still it has a pretty long life span. The problem is that most of us don't use it every day. So you buy way more than you need and then what?

From the bacon grease conversation " When in doubt, throw it out". It's not really wasting it if it has a potential for harm. And if I recall it's not a great monetary loss.
I buy a new one every six months and replace the bottle in the fridge. I have never used an entire bottle.

If it's older than 7 or 8 months, I'd throw it out. Another way you can tell if it's good or not, is by the color. It should be an amber shade, rather than the color of soy sauce. I'm with drbabs on this.I'd throw that bad baby out.

Isn't this kind of like going through your closet and finding a red belt that you haven't worn since the 80's? You didn't use it for, well, forever, and you're not sure if it's any good. Why would you start now? See the conversation on this site about bacon grease for discussion of toxins that you can't see, smell or taste. Or you could follow the advice that was given to the person who found old Crisco and call the manufacturer. But if it were me, I'd throw the stuff out.