Maybe it's because I'm a "Newbie", but I'm pretty sure I don't have that option available.
It's good to know that it exists, though. Thank you.
EDIT: Ok, found it in this post. I'm guessing it's limited by time (which I personally dislike).
I don't think it times out. I think it's that you can't edit
any more once there's been a reply beyond it. Seems
like a good thing for contentious boards, to keep you
honest, unable to alter what you've said once it's been
challenged by a reply. Otherwise, in friendly territory, I
agree that it's annoying.