My Silk must hold the record. I've had an opened carton last a month before. When it goes bad, there's no question about it. It turns into extremely foul sludge-- and it does it overnight. If it looks and smells at least sort of OK, it's probably fine.
PS: That's one of the many everyday advantages of being vegetarian in my book-- so many of my staple foods last a long time both on the shelf and after being opened. Yay for no wastage!
And I feel much more comfortable using a rule of "looks OK, smells OK = is OK" than I ever would with animal products.