Setting an 18-year limit is in line with how long the average United States Supreme Court justices has historically served on the court. This means presidents would be able to predictably appoint two justices per term, with no single president able to appoint a majority of justices on the court.

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https://www.forbes.com/sites/alisondurkee/2023/10/25/bipartisan-legal-scholars-urge-supreme-court-to-impose-18-year-term-limits/?sh=2a53bf1072fb