I haven't found any real solution to this either. However, I've noticed:

If you click "Remember Me", when logging in, it lets you idle longer

I've never been logged out in under 20 minutes :s

The reply box auto saves periodically. I've never lost post content? But I'm paranoid, so I usually copy everything before I walk away from the pc, anyway. Then if something's missing after getting logged out I just paste it.
Then again, I usually type in word or notepad and paste into the message area anyway - on any forum...

I wish I had a solution for you, but in the meantime, you might try one of these techniques so you don't lose anything?

Good luck!