Justice                Born    Age  Appointed by      Seated     Served
-------                ----    ---  ------------      ------     ------
John G. Roberts Jr.  1/27/1955  51  George W. Bush   9/29/2005   4 months
John Paul Stevens    4/20/1920  85  Gerald Ford      12/19/1975 30 years
Antonin Scalia       3/11/1936  69  Ronald Reagan    9/26/1986  19 years
Anthony M. Kennedy   7/23/1936  69  Ronald Reagan    2/18/1988  18 years
David Hackett Souter 9/17/1939  66  George H.W. Bush 10/9/1990  15 years
Clarence Thomas      6/23/1948  57  George H.W. Bush 10/23/1991 14 years
Ruth Bader Ginsburg  3/15/1933  72  Bill Clinton     8/10/1993  12 years
Steven G. Breyer     8/15/1938  67  Bill Clinton     8/2/1994   11 years
Samuel A. Alito Jr.  4/1/1950   55  George W. Bush   1/31/2006   1 day

According to this, Supreme Court Justices serve on average about 15 years -- but that figure is somewhat misleading, according to this paper, dated April 2005, by two Northwestern University Law Professors:

From 1789 until 1970, retiring Supreme Court justices had served an average of 14.9 years. The justices who have stepped down since 1970, however, have served an average of 25.6 years. This means Supreme Court justices are now staying for more than 10 years longer on average on the Supreme Court than they have done over the whole of American history.

The reason for this is not hard to find. Recently, the average age at time of appointment to the Court has been 53 years old, which is the same as the average age of appointment since 1789. The retirement age, however, has jumped from an average of 68 years old prior to 1970 to 79 years old for justices retiring since 1971. Two of the current justices are in their eighties, two are in their seventies, and four more are 65-69 years old. Only one justice, Clarence Thomas, is less than 65 years old.

For this reason, the authors advocate a Constitutional amendment limiting Supreme Court Justices to a single 18 year term.

With Supreme Court justices now staying 10 years longer than they have historically, vacancies on the Court are opening up a lot less often. Between 1789 and 1970 there was a vacancy on the Court once every 1.91 years. In the 34 years since the two appointments in 1971, there has been a vacancy on average only once every 3.75 years.

We think this state of affairs is unacceptable. No powerful government institution in a modern democracy should go for 11 years without any democratic check at all on its membership. Nor should powerful officials in a democracy hold office for an average of 25.6 years with some of them serving for 35 years or more.

No other major country in the world allows the justices of its highest constitutional court to serve for life without a mandatory retirement age. England has a mandatory retirement age, and France, Germany, Italy, Spain and Austria all appoint their equivalents of our justices for a fixed term of years. In addition, none of the 50 States appoints its Supreme Court justices for life. The federal system of life tenure for Supreme Court justices is an oddity among the countries of the world.

[...]

For 180 years until 1970, we had Supreme Court tenures of about 15 years, a practice that worked well. Now that this system has broken down, action is needed to restore the status quo. We propose a constitutional amendment instituting reasonable term limits for the members of the Supreme Court.

Because any reasonable version of such an amendment (including the one suggested in this paper) wouldn't apply to sitting justices, there's nothing to stop us from looking forward to 25 or 30 years (or more) of service from Roberts, Alito and Thomas, along with perhaps 10 years from Scalia, which should give them plenty of opportunity to wreck havoc.