If I may, perhaps we can consider worlds "complete" insofar that we can consider reality so.
At the end of the day a world is a stage upon which stories may happen. In our world, human history and the life of every last thing that exists is a story that happened in it, but in the world to us a lot of that is in the past and history. History is as much context to us as geography and politics, and that's what the world provides: context.
So since you can always tack more and more to a world to "patch" it, as you have all put it, it comes down to when you think you would be comfortable with allowing others to place their stories in it. If there's a big rich history in your world that precludes people owning machinery, you would want that written down and noted before you wanted someone's story taking place, right?
Personally, I agree with all of you; that a world is never "done," but it can still be "complete." If I haven't gotten around to explaining how the cycle of constituent mana preserves magical integrity and spell abilities, then I'll do it eventually, but it's not exactly prudent information to most stories that would take place within it.
So, I suppose to answer the question of the thread for myself, I would have to say I make a point of "completing" every world I decide to put down in words, and I personally don't write it at all until I can complete it in that sitting. I just kind of sit on it and build it in my head until I think it's ready. After that, I'll usually let it go, unless I feel the urge to come back to it and add some more minutiae to it. Sometimes I might introduce my own stories into it for the lulz, but not as often as I'd like.
