The amount of idle time varies. I've run into this same thing, losing posts, too,
I'll pass the information along and see what can be done. On behalf of the LH staff, all apologies for the inconvenience.
From the forum administrator:
"First and foremost, I would recommend selecting "keep me logged in" when signing into the site. If it still times out, then you may have some bad cookies and I would suggest clearing the cookies on your computer."
The time you spend typing & editing a post is considered "inactivity" and the clock starts ticking. You should have ~15 minutes, but glitches do occur. The vbulletin platform is good, not perfect. Copying your un-posted message can prevent loss, should your session time-out. If you do this, then the worst you'll have to deal with is simply pasting the message once you log-in.