Here's an interesting bit of trivia for you. What Hillary Clinton hopes to do in 2016 — replace a sitting Democratic president who was elected to exactly two terms — hasn't been done since 1837, when Martin Van Buren replaced Andrew Jackson.
The graph at right shows the back-and-forth between the parties over time. The Jackson-to-Van Buren transition, which came only a few decades into the life of the country, is what Clinton wants to duplicate.
We have to note the point made exceptionally well by XKCD in 2012. With only 44 presidents in America's history, the uniqueness of any particular thing is a bit diminished. It's a really small sample size. There have been back-to-back Democratic presidents only four times, including Jackson-Van Buren. And Harry Truman took over for Franklin Roosevelt, who, of course, served more than two terms.
Nonetheless, the last time the nation went from a two-term Democrat to another Democrat will have been 180 years from the inauguration of Hillary Clinton in 2017, should she win. (Has anyone in the comments pointed out the contested 2000 election yet? If not, please feel free to do so.)
For Republicans, who've been more successful at winning back-to-back presidencies, the feat was accomplished more recently: in 1989, when George H.W. Bush replaced Ronald Reagan.
A word of caution for Clinton (or for any Democrat seeking to replace Obama). Both Van Buren and George H. W. Bush had something else in common: They served only one term.