You may retire as faction leader at any time by selecting "Retire" from the Game Menu. This ends the game and computes your Alpha Centauri Score.
You must also retire if you reach the mandatory retirement year for your game or scenario. In standard games, the mandatory retirement year is M.Y. 2600 for Citizen, Specialist, and Talent levels, and M.Y. 2500 for Librarian, Thinker, and Transcend levels.
You will be warned 20 years before you reach mandatory retirement.