I get weekly deliveries of a half gallon of milk but in the past I would commonly buy a gallon trying to drink it in a 12-day window before the stated date of expiration. Beyond a quick glance or a sniff test, when is it time to chunk the carton?
A simple internet search shows, "If it is more than six days beyond the stated expiration date, you can safely assume that the milk is spoiled (or well on its way to being spoiled). Although milk can remain edible far beyond the expiration date in some instances, this is the earliest stage at which the milk might become unsafe to consume. Other factors that affect the freshness of the milk is the temperature at which it is stored and improper handling, such as drinking directly from the carton (gross!)."
Live. Eat. Drink Milk.