As of now, 10 years max - and "vice president" under these circumstances?? That fuker's lucky if it doesn't get firing-squadded with the rest of 'em!
American presidents are elected for four-year terms. While the 22nd Amendment limits presidents to two full terms in office, it also allows them to serve two years at most of another president's term. So if a president died, resigned, or was impeached and removed from office, the vice president would be sworn in. If two years or less was left on the previous president's term, the new president could serve out that term and still be qualified to run for two full terms of their own. That means the most any president can serve in the White House is 10 years.
The 22nd Amendment limits a president’s number of terms to two. However, if a vice president takes over for a president with less than two years of his term remaining, that vice president is allowed to run for two more terms, for a total maximum of 10 years serving as president. This has never happened in U.S. history.
https://www.reference.com/history/many-years-can-president-serve-21956ab657b673fd