I enquired about session timeout setting from my hosting service provider, and got this easy option that works like a charm. You need access to the .htaccess file however, but most people have it through FTP. Otherwise send a mail to your provider to add the line. The "0" is infinite. Any seconds can be added.
Quote from my service provider:
The session.cookie_lifetime value can be set in your .htaccess file. Simply create or modify the existing .htaccess file in the root of the relevant site and add the following line:
php_value session.cookie_lifetime "0"