By my quick and dirty research, the longest in history, unless someone digs up an obscure king of somewhere.
The longest reigning Monarchs are...
Franz Joseph I. Reign length: 67 years, 355 days.
K’inich Janaab Pakal. Reign length: 68 years, 33 days.
Johann II. Reign length: 70 years, 91 days.
Bhumibol Adulyadej the Great. Reign length : 70 years, 126 days.
Elizabeth II. Reign length: 70 years 127+ days (and counting)
Louis XIV was King of France for 72 years.
Ramesses the great ruled Became Pharaoh at (I think) 20 and lived until 92. He reigned 72 ysars.