While the various options for permissions and sharing permissions are many, this article deals specifically with the Free/Busy Time Privacy. For in depth details, see this video: http://youtu.be/8MsBWk0p7_M?t=12m50s.
Step-by-step guide to removing the ability to view your Free/Busy time
To remove access to your Free/Busy time:
Keep in mind, this is one of many calendar permissions options available. Removing the default Free/Busy time will prevent anyone within the organization from seeing your Free and Busy time.