To increase the phpMyAdmin Session Timeout, open config.inc.php in the root phpMyAdmin directory and add this setting (anywhere).
$cfg['LoginCookieValidity'] = <your_new_timeout>;
Where <your_new_timeout> is some number larger than 1800.
Note:
Always keep on mind that a short cookie lifetime is all well and good for the development server. So do not do this on your production server.