This contrib got me where I wanted to go, but was not done really well...
(needs other contrib? Database changes? Add commented-out code?)

Just add the following line to admin/includes/configure.php (or admin/includes/local/configure.php):
define('ADMIN_SESSION_LIFETIME', 5400); // - Zappo - Session Lifetime in seconds for Admin side.

Then in admin/includes/functions/sessions.php REPLACE (around line 20):
$SESS_LIFE = 1440;
WITH:
// BOF - Zappo - ONE LINE - Added Session Lifetime setting (admin/includes/local/configure.php)
$SESS_LIFE = ADMIN_SESSION_LIFETIME;


That's all folks!
Just change the setting in the configure.php to the amount of seconds you'd like the session to last.
File contains same instructions as above...

More...