I've never used the drafts feature.
However, I did some research and it looks like the system saves them for 24 hours, unless that setting is changed.
Past 24 hours, it deletes the draft.
My understanding with drafts is if you go to the forum you were posting in and start a new post, if you have a draft saved there (from the last 24 hours), that draft will load.
e.g., start a post in Relationships, leave the forum while you go get dinner, then come back later and it'll be there.
I just tried leaving with this post, a response to this thread... when I click the "Post thread" button in Off Topic, it's a blank post box, but when I return to this thread the draft loads (or at least, what the system saved before I left. I think it saves every 20 seconds, unless you click the save draft button yourself).
My suggestion would be if you have a long post you're writing, that you aren't certain you'll complete today, click the "toggle BB code" button, save the raw source code (so any formatting you used -- bold, links, colors, lists, etc. -- carries over), and you'll have your draft to work from later, even if 24 hours have passed and the system dropped it.
Chase