For better or worse, I have had that happen to me multiple times as I find my self adding and editing my posts and comments throughout the day - in between classes, bus rides and whatnot. Eventually, it gets to a point I'm sure it'll time out... or I'm only satisfied with the post by the time I get home.
My solution(s) as of yet are composed of a couple things:
1. Write it in a word editor: MS Word, Notepad, etc.
2. Select the whole post and copy it to my clipboard (when done or want to save my work primitively).
3. Both. I'll be stubborn and continue typing into the comment box and copy/paste what I have into a word editor. Then mess around with the piece and paste it back into the submission box.
I might be going a bit nuts, perhaps it's because (and I'm inferring here from mk's comment) the save draft option is server side, but I swear it's not always there.
Hope this helps for future issues.