For security reasons, the system is set to time off and invalidate your session if you have not used the application for 30 minutes - to avoid this, you simply need to click any link or refresh a page which will reset the timer.
A countdown message will appear, informing the user of how long is left until the session times out.
To amend how long the system has until it times out due to inactivity, browse to:
Administration>Company Settings> Security Settings
This controls the length of time after which the system will log you out if you do not make any actions- By default this is set to 30 minutes and it has a maximum value of 240 minutes (4 hours).
Enter the number of minutes and click submit. You will need to logout and back in for this to take effect.