In the above figure choose "Private Computer" when you are using your own PC. This will ensure that your session does not expire. In case you are using a public computer, choose "Public or shared computer". In this case your session will expire in 20 minutes. While you are typing a long message in the compose window, the time frame of typing is NOT counted as activity. Hence it is recommened to copy the text in the buffer (Press CTRL + A to select all the message and then press CTRL+ C to copy it in the buffer). If upon clicking  "send" you find that your session expired just relogin and compose and press CTRL + V in the message area to paste the contents of the buffer.