Not 100% sure about this...I think if there is no custom session cookie name in effect, then the session_expire configuration option is ignored, and the default in the php.ini is used instead.  The session_expire config option is used to set the timeout of the session when the session is initialized...just checking libraries/icms/core/Session.php now...

Yeah...from the updateCookie method in there:

($icmsConfig['use_mysession'] && $icmsConfig['session_name'] != '')
					? $icmsConfig['session_expire'] * 60 : ini_get('session.cookie_lifetime')

I'll add that in and then commit this...