When the U.S. Constitution was modified, after FDR's tenure, it limited any president to 2 elected terms, meaning eight years. There's a caveat, however. If said president stepped in as an unelected president for no more than two years prior to those two full, elected phrases, that is also acceptable. For example, if an acting vice president have been sworn in as president greater than two years right into a presidency, they might be elected for a further eight years, totaling 10.