My dad doesn't believe in throwing away food. He'll eat it up until the point it absolutely cannot be eaten any more. So however long after the sell-by date that is is my answer.
I am anal, and everything in my fridge or cupboards gets thrown out on the expiry date. No exceptions. That said, I never have milk last until the expiry date, it is always gone before then so I never have to worry. If there is one thing in my house that does not get watsed, it's Milk!
Once opened, no longer than a week. No matter how much is in the container, in the garbage it goes. (This is why I started freezing extra milk) Once it hits the expiration date (opened, unopened) it automatically goes in the trash. I'm not taking any unnecessary risks with my food.
Milk stays in our fridge as long as it's still good. I think we once had a gallon of milk last for almost a full month after the printed expiration/sell-by date, and it was still fine to drink on it's own. Basically if it still smells ok, and doesn't make you throw up on contact with your mouth, it stays till it either goes bad or gets used up.
I drink milk until it has it's own personality. It's glorious and nutricious and mighty expensive. The Stuff always looked tasty @V@
Hey, here's a concept. Transformers brand milk. Leave it in the sun for 36 hours, with your choice of Autobot or Decepticon insignia of course, and, Voila! It transforms into Yogurt. We could make millions!
I go based on smell. Milk that's still good has a very clean, almost imperceptible smell. Once it starts to turn not much in this world smells more foul to me.