How Long Do Senators Actually Serve?

Today is the 2015 State of the Union address by President Obama. His speech has already been widely written about, and as with most SOTU addresses presenting it is largely a formality. There are many traditions associated with the SOTU, including the POTUS having to be invited to give the talk.

Another SOTU tradition, which I find very American (and started in the 1960's), is the SOTU response given by a member of the opposite political party to the POTUS. This year Joni Ernst, a brand new Senator from Iowa whose campaign was dominated by hog castration, will be the 16th woman to deliver a SOTU response.

Senator Ernst may be a public figure for quite some time, so I wondered today:
how long do Senators typically serve? 
There are some very well known examples of career Senators, for example the lengendary badass, Daniel K. Inouye. But what of the typical Senator? Here are some numbers...

After nearly 230 years, the US Senate has more than 1855 distinct former members. The term length for Senators is 6 years (compared with the typical length of 2 years for representatives).
Here is the histogram of term durations for all past Senators. Note: some people served multiple, non-consecutive terms, or changed parties, which counts in this data as a separate term. You can clearly see the spikes of 1, 2, ... 6 terms. The average term is 7.6 years (median 6 years) for all past Senators.

However, the typical duration a Senator serves appears to be changing. Above I have plotted the duration versus the year elected for all past Senators. You can see in the running 2-year median the typical serving duration has increased, especially since WW2. While the overall average for length served is 7.6 years, since 1950 it has risen to 11.8 years. This means on average Senators are elected twice, and will serve during at least 2 presidencies.

This is longer than the average rein of a Pope (7.3 years), but shorter than a US Supreme Court Justice (16.4 years).

GitHub repo with data is here

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