How long will margarine be “good” once it’s opened?

Once opened, margarine usually lasts about a month in the refrigerator. Keep a margarine stick in a covered butter dish to decrease drying out and prevent exposure to odors from other foods.
 
When buying margarine in a tub, select a tub size that you’ll finish in a month. Try to use a half-inch a week so that you’re always using a relatively fresh surface unhardened by oxygen.
 
Source(s):
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin