I got logged off from the site when I was trying to write a long letter.
We have to balance convenience and security. Some guys are accessing the site in public places such as a library or their cell phones. If they don't log off, someone else can get into their account, etc. So that's why we have the automatic logoff after 30 minutes. If you have a long letter just open a word document to write it first, then copy and paste it on the compose letter screen.