The WordPress wpDirAuth plugin currently has a hard coded session time of 1 hour for directory authenticated (LDAP etc.) users. Hopefully at some point in the future this will become configurable. Discussion here.
On a related note, inserting
define( 'AUTOSAVE_INTERVAL', 60 ); // Seconds
in wp-config.php changes the autosave interval (default is 60 seconds).
Edit: Fixed in V1.9.3 thanks to patch submitted by Sean Leavey – time is now configurable.