So, yeah. Zoe is awesome in her return to House of Cards. But, that doesn't necessarily mean she's going to come back time and time again for my personal pleasure. In fact, Kate Mara is only in the one episode of House of Cards Season 4.
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.
A Senator's term of office is six years and approximately one-third of the total membership of the Senate is elected every two years. Look up brief biographies of Senators from 1774 to the present in the Biographical Directory of the United States Congress.