Vidugavia established his realm to the east of Greenwood the Great about this time. We know that he had a line of successors, and it seems that they continued to style themselves 'King of Rhovanion' after Vidugavia's time. The use of the title, at least by the direct line, could not have continued past III 1856, when Vidugavia's descendant Marhwini led his people out of Rhovanion to resettle in the Vales of Anduin.
This is confused somewhat by a reference to Eärnur's arrival at Mithlond in III 1975, when he was stated to have with him 'proud princes of Rhovanion' (The Lord of the Rings, Appendix A, I (iv)). These could not have been direct descendants of Vidugavia (whose realm had ceased to exist more than century before) so perhaps some other line claimed the Kingship after Marhwini abandoned Rhovanion.