I would make the max number of drafts and days configurable, default to 1000 and 30 days perhaps. Storage is cheap.
I checked a while ago, and 100 drafts lasts me about a day. I preview and edit a lot.
I sometimes use the drafts to restore a lost post, but it's quite useless as it doesn't last long. I'd even want more than 1000 posts (and 30 days) to be more useful. I currently have an unfinished post that I've been postponing for about a month. Once in a while, I click "Preview" to keep a new draft again, but I've also saved a backup on my PC. For convinience, it would be good if drafts are more reliable.
If you close the window, pc restarts, bsod etc and open a thread you are prompted if you want to load the draft. Thing is after if you write in another thread it overwrites the existing draft.
Most of the time, my browser restores all tabs and the unfinished post is still there. But not all the time, and I've lost some posts that way.