14 days sounds like a good compromise. 2 weeks is easy to remember.
While part of your suggestion aims to fix the top of the scoreboard, it affects everyone. One concern on CC's behalf is that the active members might drop significantly, which would look kind of bad, even knowing the reason why (which a lot of people might not). I don't think, therefore, having a scoreboard that more adequately reflects the current active users would be an implementable suggestion.
What if this suggestion were modified to only target the Conqueror title? For example, if you reached Conqueror, you have XX days (fewer than 30) before you lose the title due to inactivity. You remain on the scoreboard, but are only the highest field marshal, even though you have the most points.