/usr/share/phpmyadmin/config/config.inc.php:
$cfg['Servers'][$i]['LoginCookieValidity'] = 86400;
OR
/etc/phpmyadmin/config.inc.php:
$cfg['LoginCookieValidity'] = 86400;
/usr/share/phpmyadmin/config/config.inc.php:
$cfg['Servers'][$i]['LoginCookieValidity'] = 86400;
OR
/etc/phpmyadmin/config.inc.php:
$cfg['LoginCookieValidity'] = 86400;