I was talking to sado about this the other day. I was saying that I could work on a map forever if I wanted to, and that was just polishing the appearances.
But to answer your question: I believe that a map is finished when the following are true:
- There are absolutely no neglected areas when it comes to appearance. That's not to say that all areas need to be spammed with objects. Instead, no area should be left completely flat and with solid, uniform-facing tiles.
- Locations and resources are balanced accordingly.
- The battlefield is conducive to proper game play
Now, all three of these items could be improved upon if if they are technically finished. Just like you might complete a project for your boss and tell him or her that it is finished. There may have been a detail or some that you could have refined more. Perhaps someone else could have done them better. Yet, the project serves its purpose and can be used to that end.