At older Limesurvey versions (like 2.0) that field was always shown at the "Global Settings". For newer versions the field is only shown if DB sessions are used. See this note at the manual: "This setting is only available when LimeSurvey is configured to store sessions in a database. When using regular, file-based sessions, it is up to the system administrator to define the right values for "session.gc_maxlifetime", "session.save_path", etc. in the PHP configuration."

The setting is not available when using file-based sessions. There are several reasons for this:

• a local setting can be overwritten by the webserver
• other applications may use the same setting with lower values which also overwrites such a Limesurvey setting

I have added some more hints to the manual.