Why is my soymilk going off so fast?
Expiration dates on milk are not intended for open containers. Any open milk is only good for 3-5 days. After that, it is not safe for drinking, and taste is not guaranteed either. 

Many people seem to not know that. Traditionally, you could only buy HTST pasteurized milk, which has 7-10 days of usable time altogether. It spent 2-3 days on its way to the customer, and if a milk opened on day 3 after production went off on day 8, nobody wondered, because it was on or just before the expiration date. 

Today, you get also ESL milks and nut milks, which have a much longer life in the closed container. But once you open them, they start acting like any other food - they are only good for 3-5 days in the fridge, no longer. The expiration date doesn't matter for open foods.