Anyone who studies American history knows that our only President who had more than two terms (eight years) in office was Franklin D. Roosevelt, who actually was elected four times, and served a total of 12 years and 39 days before dying in office in 1945.
But there were others who could have had more than eight years in office, were it not because of their own decision not to seek another term, or due to constitutional limitations via the 22nd Amendment!
These potential cases of Presidents who could have had more than eight years in office include:
Andrew Jackson (1829-1837), who would have won a third term had he chosen to run, but instead his Vice President, Martin Van Buren, ran and won the Presidency.
Theodore Roosevelt (1901-1909), who served seven and a half years after succeeding William McKinley six months into his second term, and then chose not to run in 1908, backing William Howard Taft who won, and then challenging Taft in 1912, on a third party line (Progressive Party), but lost to him. Despite the loss, TR won six states and 88 electoral votes, the best third party performance in American history.
Calvin Coolidge (1923-1929), who served five and a half years in the Presidency, after succeeding Warren G. Harding after two and a half years in office, and decided not to run in 1928, and instead, we saw Herbert Hoover win the Presidency.
These three Presidents mentioned above were popular enough to have won another term, and in each case, would have ended up serving more than eight years in office, as FDR did!
And then there are four Presidents since the 22nd Amendment limitation of two terms or ten years in office if succeeding to the Presidency with less than two years left of the term when they became President, all of whom could have been elected to another term, had there been no such limit!
Dwight D. Eisenhower could have won and run a third term in 1960, as could Ronald Reagan in 1988, and Bill Clinton in 2000, while Lyndon B. Johnson, had he not dropped out in 1968, likely would have beaten Richard Nixon, since his Vice President, Hubert Humphrey, came close to doing so, and did not have the fact of being President to help him win the election!
It is interesting that in all cases mentioned except three—Eisenhower, Johnson, and Clinton–the party of the President who did not run for reelection won the election. Eisenhower saw Richard Nixon lose a close election, despite much evidence of a fixed result for John F. Kennedy in 1960, and Johnson saw Humphrey lose to Nixon in another close election, where LBJ would likely have turned the tide! And Al Gore lost in 2000, despite a popular vote majority, due to the intervention of the Supreme Court in 2000, giving the Presidency to George W. Bush!
So instead of one President with 12 years and 39 days in the Presidency, we could have had, additionally, Andrew Jackson, Dwight D. Eisenhower, Ronald Reagan and Bill Clinton with 12 years in office; Theodore Roosevelt with 11 and a half years in office; and Calvin Coolidge with nine and a half years in office and Lyndon B. Johnson with nine years and two months in office!
And Martin Van Buren, William Howard Taft, Herbert Hoover, John F. Kennedy, Richard Nixon, George H. W. Bush, and George W. Bush might never have been President if the Presidents before had sought or been able to seek a third term in the Presidency!
PS Another thought that has come to me, belatedly, is that Grover Cleveland (1885-1889, 1893-1897), the only President with two nonconsecutive terms, actually won the popular vote in 1888, but lost in the Electoral College. Had the result been different, Cleveland, in theory, might have run in 1892, anyway, and could have been a three term President, and Benjamin Harrison would never have been President!