How long will butter be good once it’s opened?

If left in its partially-open original paper wrap, stick butter will start to harden and smell stale after about a week, as oxygen sets in. You can store butter in an airtight container in the refrigerator (40° F) for 2-3 weeks.
 
Source(s):
Assistant Professor Franco Milani, Department of Food Science, University of Wisconsin
National Dairy Council Factsheet: “Butter”
The Microbiological Safety and Quality of Food, by Barbara M. Lund and others, 2000.