How long will a carton of milk be “good” once it’s opened?

Stored at or below 40° F, an open milk carton will be fine in the refrigerator for 5-7 days, usually longer, said Cornell University food science researcher, Steven Murphy.  However, many factors, such as processing date and method and exposure to light and heat, influence how long a particular carton will remain “good.”   Generally, high fat milk products don’t keep quite as long as fat-free, says food scientist Susan Brewer.
 
Source(s):
Steven C. Murphy, Senior Extension Associate (Milk Quality Improvement Program)
Department of Food Science, Cornell University
 
Susan Brewer, Ph.D. University of Illinois, Department of Food Science and Human Nutrition

National Dairy Council Factsheet: “Newer Knowledge of Dairy Foods--Protecting the Quality of Milk and Other Dairy Foods”