The average length of a Justice's tenure in the U.S. courts can vary significantly depending on the specific court in question. Here, we'll focus on the federal courts, especially the U.S. Supreme Court, as federal justices tend to have longer tenures compared to state court judges.

1. U.S. Supreme Court Justices: Justices of the U.S. Supreme Court serve lifetime appointments, as outlined in Article III, Section 1 of the United States Constitution. This means that they can remain on the bench until they choose to retire, face impeachment and conviction by Congress, or pass away. Consequently, the length of tenure for Supreme Court Justices varies widely. Some serve for a relatively short time, while others have significantly longer tenures. Some of the longest-serving Supreme Court Justices in history include William O. Douglas, who served for over 36 years, and John Marshall, who served for over 34 years.

2. Federal Appeals Court Judges: Judges on the federal appeals courts, also known as circuit courts, are appointed for life as well. However, the average length of their tenure can vary depending on individual circumstances and career choices. Many federal judges serve for several decades, while others may choose to retire or take senior status (a form of semi-retirement) after a certain number of years.

3. Federal District Court Judges:Federal district court judges are also appointed for life, but their tenure can be influenced by several factors. Some may serve for many years, while others may retire or take senior status earlier, often based on personal decisions or external factors.

It's important to note that while these federal judges serve lifetime appointments, many choose to take senior status at some point. In this status, they have a reduced caseload, allowing them to continue to contribute to the judiciary but with more flexibility.

In summary, the average length of a Justice's tenure in U.S. federal courts is influenced by a range of factors, including personal career choices, retirement decisions, and external circumstances. Supreme Court Justices serve for life, but their tenures can vary widely, while lower federal court judges also serve lifetime appointments but may make decisions about retirement or senior status based on individual circumstances.