Changing php.ini will change the session duration for all the websites running on the server. To change it just for PhpMyAdmin, open
config.inc.php and add:$sessionDuration = 60*60*24*7; // 60*60*24*7 = one week
ini_set('session.gc_maxlifetime', $sessionDuration);
$cfg['LoginCookieValidity'] = $sessionDuration;