In days of much lower life expectancies, Henry I, Henry II and Edward I all reigned for 35 years and Edward III for 50 years. James I/VI was King of Scots for 35 years and King of England and Scotland for a further 22 years.
According to the Guinness Book of Records, the longest-reigning monarch ever was a 6th Dynasty Egyptian Pharoah, Pepi II, whose reign of 94 years is unlikely to be surpassed. However, he was apparently a very weak ruler.
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