Yeah, that irritates the life out of me too when I go to post something and the site has timed me out.
At least for me, if I click "Remember Me" when I login, I never get timed out. I started a thread and left the browser open for 8 hours. When I returned, I clicked "Post" and I didn't lose anything.
If you're not using "Remember Me", there is a 15 minute time-out.
Regards,
Chris