I voted Never but rarely would be more accurate. I have 2 drafts from 2012 and that's it. Other than I used it as I was writing this to confirm what I remember about the functionality.
Honestly I don't know if I've ever actually used it as intended other than this one time now as I'm reply to this topic, i.e. to save a post that I will come back to later and finish. Instead I've used it with the intent to do so but then forgot that I had saved the draft, or just decided to start over. As it currently stands, it's a bit clunky and not very intuitive, and the fact that you lose data (namely, formatting and attachments) makes me unlikely to really use it on purpose.
Suggestion based on what I see in other software: If you are working on a post, autosave it as a draft periodically (e.g. every minute). The autosave should save any content, including formatting, poll options, attachments, etc. on a per topic (for new reply drafts) and per forum basis (for new topic drafts). Then if I return to that forum or topic and go to the posting page, it should automatically load it back in, or at least ask me if I want to. Maybe have a "drafts" box on the new topic/new reply page that lets me select from a list of saved drafts. Then, autosaved drafts can last a configurable amount of time, e.g. 24 or 48 hours and then get pruned. Or the user can intentionally save the draft (instead of letting it autosave) and it would be more permanently saved.
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