Tom is elected President, Carl becomes VP. Tom dies with less than two years of his term left, Carl takes over. Next term, Jim is elected, Carl becomes VP again. Jim dies with less than two years of his term left …
You can repeat that as often as you like (as long as Carl lives long enough). From the wording above it seems that time served as President in different terms does not add up to the two-year threshold.
After that, Carl can be elected twice.
So in theory, the longest possible duration a US citizen can be president is only limited by the citizen’s lifetime.