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.
Each state, regardless of its size, has two senators. Senatorial terms are staggered, so every two years approximately one-third of the Senate is up for election. Each staggered group of one-third of the senators is called a 'class'.