Anyone who studies American history knows that the 22nd Amendment, added to the Constitution in 1951, prevents any future President from serving more than two complete terms by election or a total of ten years by succession in the last two years of the Presidential term.
Only Franklin D. Roosevelt served more than eight years in the Presidency, a total of 12 years and 39 days, having been elected four times (1932, 1936, 1940, 1944), and this fact causing the opposition Republicans, when they controlled the 80th Congress in 1947-48, to pass the 22nd Amendment in 1947, and send it on to the state legislatures for ratification.
However, Ulysses S. Grant in 1876; Theodore Roosevelt in reality in 1912 as a third party (Progressive Bull Moose) candidate; Woodrow Wilson in 1920; and Harry Truman in 1952 considered a third term.
Additionally, it is clear that Dwight D. Eisenhower in 1960, Ronald Reagan in 1988 and Bill Clinton in 2000 would have won a third term if it had been allowed and they had agreed to seek it , with George H. W. Bush being the beneficiary of Reagan in 1988, and Al Gore being the beneficiary of Clinton in 2000, winning a larger margin of popular vote victory than any of the four cases of popular vote victory but Electoral College loss!
Also, if one considers popular vote victories of Andrew Jackson in 1824 and Grover Cleveland in 1888, but in each case losing the Electoral College, that could have meant three terms for Jackson (1824, 1828, 1832) and for Cleveland (1884, 1888, 1892)!
So if things had been different, instead of only FDR having a third and fourth term, we could have had Andrew Jackson, Ulysses S. Grant, Grover Cleveland, Theodore Roosevelt, Woodrow Wilson, Harry Truman, Dwight D. Eisenhower, Ronald Reagan, and Bill Clinton having third terms in the Presidency!