Sour milk

When does milk go bad? There's not an exact moment when you can say, "right now it's ok to drink, but in an hour ... watch out!" I had cereal this morning, with milk, and it didn't taste quite right, but I disregarded it for two reasons: I knew it wasn't due to spoil until Oct. 9, and I had recently brushed my teeth, so I allowed for some Crest after-effect. So this evening I decided to do a followup on the milk, and sure enough -- BAD! Not just bad, spoil your appetite for dinner bad. And to think I had some this morning. Which leads me back to my original question: When does milk go bad?