When the 22nd Amendment was added to the US Constitution in 1951, after being passed by Congress in 1947, it limited Presidents to two complete terms of office, or eight years, OR if becoming President by succession during a term, allowing any successor who came to the Presidency in the last two years of the term, to be elected twice, but if succeeding during the first half of a Presidential term, such person could only be elected once.
Therefore, when Lyndon B. Johnson succeeded John F. Kennedy with one year and two months left of the term, he was eligible, in theory, to be elected twice, although he withdrew from running for a second full term in 1968.
No one else has had that opportunity, and since Richard Nixon resigned, and was replaced by Gerald Ford with two years and five months left in the term, no vacancy in the Presidency has occurred, now for the past 40 years, the longest such period since the first Presidential replacement in 1841.
Now that the second half of the second term of Barack Obama has passed its midpoint at 12 noon two days ago, Vice President Joe Biden has the situation, whereby IF he succeeded to the Presidency at any time from this point forward, he could, in theory, serve up to 10 years in the White House, from ages 72 to 82.
While this is highly hypothetical, it could happen, with the constant death threats against President Obama, with many of these reported threats discussed in my forthcoming book on Presidential Assassinations and Threats.
The point to be made is that IF Joe Biden suddenly became President before the 2016 primaries, caucuses and elections take place, this scenario could occur, and no one should do what is so common—to underestimate the potential of Vice President Joe Biden, who has added so much to the Vice Presidential office’s history in his six years as number 2 man in American Government.