It depends on the kingdom, really. For example, the king of En has reigned for over five hundred years, but the king of Kou has only recently fallen - it will be some time before Kou is stable again. Kei just had a new king take the throne, and so on.
That of the heavens. If one doesn't break with the mandate of heaven - protects his or her people, maintains order, and so on - one remains king. If a king abuses their power or breaks the rules set forth for monarchs, they've lost that right.
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The same king?
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