A presidential term lasts 4 years (at least, if nothing unexpected happens). This gives us an immediate result of 48 months: 12 each year.
The shortest term ever served is the one of William Henry Harrison, 31st president. Starting on the 4th of March 1841 and ending abruptly on the 4th of April of the same year, the term lasted only a month.
Franklin Delano Roosevelt was first inaugurated on the 4th of March 1933 and died in office on the 12th of April 1945: he is the longest-serving president with 145 full months. Read more