I keep mine (opened) in the fridge for 4 weeks max and then pour it out. It does get chunky, curdled, and smelly if it goes bad, but that usually takes a bit longer. One test I've learned (and this is gross, I know) is to put suspicious soymilk in a mug and microwave it for 1 minute. If it jellies up and seems solid, well, that's when to toss it.

great info--thanks! I will try freezing half of the next carton and report back. expiration dates are usually pretty conservative, so it's reasonable to expect product to last at least a week beyond. although this is why we have a nose, I guess!

I feel safe using milk and soy milk past the expiration date, just using a sniff test. I have a cold fridge and a sensitive nose. It isn't unusual fir either to last a week paste the expiration date. Same with eggs, I pretty much ignore any date on eggs. Here is a fun discussion on chowhound:
http://chowhound.chow.com/topics/486116

Why not just go by the experation date?

I've never tried it, but maybe you can try freezing some for later use? Here are some instructions: http://www.ehow.com/how_5632401_freeze-soy-milk.html

I use soymilk in limited quantities, try to buy no larger than quarts. When it is bad there is no question, and it does last far longer than the labeling would lead you to believe. Treat it like dairy milk, sniff it and eye it as you would dairy milk -- but remember, yogurt and sour cream are just spoiled milk. No, don't go against your judgement, but my experience is that the CYA instructions can be overridden.

Spoiled soymilk is pretty nasty - gets a bad smell, and tends to curdle. It seems to last well past its expiration date in the fridge. I'm confident if it looked fine and smelled fine it was fine.

Yah- that's annoying. I don't know how well soymilk freezes, but maybe freeze a portion of your next batch and then defrost it right before you need it- Either it will be good (yay) or you will chuck it, just like you would have had to anyway.

thanks, guys! it's annoying that the kind I like is available only in half-gallon containers. I think it's time to just pour it down the drain.

Yah- I agree- I use unsweetened Soy Silk and if it separates, I chuck it. It does seem to last a long time, but I usually get rid of it within two weeks of opening. If I think I won't be using it much, I generally buy it in a small container (in a tetra pak).

I did a lot of reading to try and find the answer for you. (IMHO, it's always bad...sorry.) Apparently it looks and smells bad and gets chunky like cow's milk when it's spoiled. But seriously? Why take chances? If in doubt, throw it out.

Someone answer this. We all need to know. My friend had houseguests when he was out of town. They used the very old soy milk on their cereal that was left in the fridge. It wasn't sour at all and had definitely been opened for more than two weeks.