How many years can a person be President and why?

The longest number of years a person can serve as President would be no more than ten years. Today, a President may be elected to two, four-year terms. However, if he was Vice President, and the President died or was removed from office halfway or more than halfway through his term, the Vice President would finish that term, which could be no more than two years, and then could run two more times. If the Vice President took over less than halfway through the President’s term, that would count as a full term, and he could run only one time.
There was a time when a President could serve an unlimited number of terms. However, that changed after President Roosevelt was elected four times. The Twenty-Second Amendment was added to the Constitution, which limits the President to two terms.
 
 
 
https://constitutioncenter.org/interactive-constitution/amendment/amendment-xxii