If you login to ITM and then don't do anything for 30 minutes, your session will end. This is by design - in the same way that your online banking session will expire after a short time. This helps to keep the system secure, and also preserve the server's resources.

If you use the 'Remember Me' tickbox at login, then your browser will store a cookie on your pc to allow you to log in again directly - meaning you generally won't notice if the 30 minute timeout occurs. Note that this only works if you have chosen to allow cookies in your browser.