U.S. presidents are limited to serving two elected four-year terms in the White House and as many as two years of another president's term. That means the longest any president could serve is 10 years, though no one has been in the White House that long since Congress passed the constitutional amendment on term limits.
The number of years a president can serve in the White House is spelled out in the 22nd Amendment to the U.S. Constitution, which states "no person shall be elected to the office of the President more than twice." However, if an individual becomes president through the order of succession, that is by taking office after the death, resignation, or ousting of the previous president, they are allowed to serve an additional two years.
https://www.thoughtco.com/why-presidents-only-serve-two-terms-3367979
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