Answer:
There are no terms for US Supreme Court Justices. A Chief Justice may remain in that office until he or she decides to leave or is unable to do the job any more (as in very sick or dead). If a Chief Justice steps down, the Senior Associate Justice serves in his (or her) capacity on an interim basis until the Chief Justice is able to return to work or the President and Senate choose a successor. This has happened a few times in the history of the Supreme Court.