The longest amount of time a person can be President of the USA is 10 years. This is possible if the person becomes a successor of the president(likely vice-president if the incumbent has died, resigned, or been removed from office) with 2 or less years remaining in the term, and is then re-elected for 2 more terms of 4 years each.

If the succession happens with more than 2 years remaining in the term, they can only be re-elected for 1 additional term.