[only1percent]

A person can serve up to, but not including ten years -- nine years and 364 days, to be exact. Thus, Gerald Ford, who served more than one year and 364 days of President Nixon's term, is eligible only to be elected one more time; had he won in 1976 he couldn't have run in 1980. However, Lyndon Johnson, who became President with only a little more than one year of John Kennedy's term remaining, was eligible to be elected twice: he ran and won in 1964 and was briefly a candidate for re-election in 1968 before he decided the poor progress of the Vietnam War made his re-election unlikely.

[Luke Skyfreeper]

Sorry, I stand corrected. You guys are correct. Ten years at most.

[Luke Skyfreeper]

A person can serve up to, but not including ten years -- nine years and 364 days, to be exact.

Nitpicking here, but I think you're wrong about this:

...no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of the President more than once.

So the way I read it, two years precisely, you're still eligible to be elected twice. Two years and a day, you're only eligible to be elected once.

So the possible legal limits on length of service (barring getting back in via succession after all eligible elections) range from 6 years and 1 day (or possibly, any portion of a day), to 8 years, to 10 years.