Many its a big issue, the Magento takes us to login screen after 20minutes. so when customers are working on any CMS pages or Email templates, it will not save the data, and simply go to login page when we click on save.
Here is the best solution for you Comment the following line and replace with the below line files, this way the session exists for a longer time.
Goto /app/etc/local.xml and replace the below line.
<!– <session_save><![CDATA[db]]></session_save> –>