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.
Length of terms of state senators. The length of terms in state senates in the 50 American state senates is either two years or four years. Senators in 31 states have a four-year term. Senators in 12 states have a two-year term. Senators in seven states (Arkansas, Delaware, Florida, Illinois, Minnesota, New Jersey and Texas) have terms that are sometimes two years and sometimes four years, depending on the proximity of the election to the legislative re-apportionments that occur every 10 years after a federal census.