Democrats shouldn't necessarily relax if Sonia Sotomayor wins confirmation to the Supreme Court. There's the matter of how long she'll be there.

In a NationalJournal.com analysis of Supreme Court nominees going back to the Franklin Delano Roosevelt administration, justices picked by Democrats have served about five years less on average than those picked by Republicans. Not counting current members of the court, GOP nominees have served about 20 years each, compared to about 15 for Democratic picks.

At 55, Sotomayor is close to the average of all nominees since 1937: about 53.5 years. But those closest to her in age have tended to serve less time yet. There have been 11 justices who were 53 to 57 as nominees; three are on the current court (John Paul Stevens, Stephen Breyer and Samuel Alito) and two left the court early (Arthur Goldberg to become ambassador to the United Nations, and Abe Fortas in a controversy over a past honorarium). The other six averaged about 14 years on the court.

Of course, these numbers don't take an individual's own circumstances into account, and they can't account for the many reasons why justices might step down. In 1942, longtime FDR ally James Byrnes left after 16 months to become Roosevelt's "assistant president."

But Democrats seeking good news in these stats can point to two numbers. The average age for a justice leaving the court going back to the FDR era is 71. And if you don't count Alito or John Roberts, the two newest justices, every confirmed nominee going back to Thurgood Marshall has served at least 15 years.

Editor's note: This is the third in a series of posts examining historical data from a database compiled by Northwestern law professor Lee Epstein and her colleagues. Keep checking back for more context and analysis on Sotomayor.