The $cfg['LoginCookieValidity'] setting contains the value 1440. This value are the seconds until a cookie will be invalid on inactivity but 1440 seconds are 24 minutes. Maybe the programmer thought the value are minutes instead of seconds because 1440 minutes are 24 hours which makes more sense. In my opinion this value should be set to 86400 seconds (this are 1440 minutes) or to 1800 seconds (this is the old value of 30 minutes).
This bug was fixed in repository and will be part of a future release; thanks for reporting.
I got a comment on the mailing list. I am putting back the 1440 seconds value and adding a comment. This is to match the php.ini session.gc_maxlifetime, see http://ca2.php.net/manual/en/session.configuration.php