Disclaimer: I am not speaking ex cathedra or as a Factory Judge. I dislike it when judges set up their personal standards as infallible doctrine. Consider this some unofficial advice based on a very long career of creating and judging Factory submissions.
Q: How long should a post be? What constitutes a quality post?
A: As of this moment, there is no set standard by word count (and there are serious arguments against instituting one). However, my general rule of thumb (and the one followed by most judges I know) is 190-250 words at minimum. The instant that judges see a thread made up of posts that are exactly 100-110 words apiece, they know a chunk of text is being milked for as many posts as possible. Instantly, they assume that less work is being done than is being claimed, and that may be a valid assumption. The same goes for one- or two-line posts: consciously or not, a judge instantly starts 'condensing' the thread, or estimating how long it would actually be if condensed down to posts of a respectable length. IE, figuring out how much work actually got done.
Q: What about re-using dev threads?
A: Currently, there is no posted standard on this that I'm aware of. In general, judges will allow a dev thread to be used for a component and then for one thing that uses the component. But using that component for more than one larger submission, and expecting that dev thread to cover it, could wind up looking like cheating. There's only so long you can milk a dev thread. It's a very good idea to keep a private list of what threads you've done and when you've cited them as dev threads for submissions.
Q: What about extremely long threads?
A: If possible, give a range of relevance ('posts 45-63 show Bob and Joe fighting to obtain the Kalamazoo Amulet'). As above, there's only so long you can milk a dev thread.
Q: I don't have the attention span to keep a list. What do?
A: I strongly suggest citing your dev threads using the name of the thread as the text of your hyperlink.
  • Good: Google - (description, why it's relevant to the sub)
  • Not So Good: [x] - (description, why it's relevant to the sub)
This allows you and your judges to search the subforum for the title of your thread, and make sure you haven't already cited it five times by accident. I wish I'd started doing this three years ago. I highly recommend listing the thread name when citing huge rafts of dev for company tier upgrades. I've even found multiple cases where two different companies got high-end tier advancements off the same major project threads, just because people forgot what work they claimed and when they claimed it.
Hope this helps streamline your Factory experience.