According to Knowledgesbase solution 50290, for system administrators, your account will auomatically log you off after 15 minutes of inactivity.

Working on an email or updating a part, does not qualify as "activity", even though Knowledge base solution says it does.  So after 30 minutes of working on a part, you click save and Boom, back to the log-in screen and all work is lost.

Please have an auto-save or some other recovery mechanism when working on parts and emails!