How long may one serve as President?

A Presidential term is four years, and no person shall be elected to the office more than twice, and no person who had been appointed President (to replace a President who had been previously elected, but unable to serve his full term) for more than two years shall be elected more than once. In theory it would be possible for someone to serve almost ten years, if they were elected for two additional terms after serving a two year mid term appointment.