Once opened, how long will milk / lactose-free milk remain usable?

The moment the carton is opened, milk becomes exposed to microorganisms, but, provided it’s stored in a refrigerator, an opened carton of milk that hasn’t passed its “sell by” date should last about 5-7 days, possibly longer.   
 
An open carton of  ultra-pasteurized milk, including lactose-free milk, deteriorates at a rate similar to conventionally-pasteurized milk, according to the FDA. Brands may vary. McNeil Nutritionals recommends consuming refrigerated cartons of its Fat Free LACTAID® milk within 7 days of opening.
 
Source(s):
The FDA Center for Food Safety and Applied Nutrition’s A-Z Reference Guide
Jennifer Minder, Manager, Communications, McNeil Nutritionals, LLC (produces LACTAID® lactose-free milk)