"The Havdalah service marks the end of Shabbat. It should be performed no earlier than nightfall on Saturday night. Nightfall is the time when three stars can be seen in the sky. It is normally about 45 minutes to an hour after sundown, depending on your latitude." (jewfaq.org)You can check what time Shabbat ends on the Chabad.org Calendar ... just wait until after Shabbat to look it up.