As the US president is more or less in the same position as the monarch in the UK -- the main *official* task is to approve or reject legislation already passed by parliament -- you should perhaps look at the 63 years 216 days Victoria reigned, or 70 years 214 days of Elizabeth II, both far exceeding any recent US, Chinese, or Russian supreme leader.

I recently noticed with horror that my birth is now closer to the end of Victoria's reign in 1901 than it is to today.