I thought it was ever thirty days for duchies. But still...
I would agree that the rules are inconsistent. Not sure what the result should be. Duchies should not be constantly renamed. Especially not just because a new duke was appointed.
But at the same time you should have the opportunity to name your char a semi-god walking the earth and have all worship him as higher being. Then the realm (new realm) and Duchy should carry his name also prob.
Why not limit the rule to changing Duchy names to 2 months or so? That should solve the issue no?