What us president served for six years?

In order to serve 6 years, a president would either have to spend two years completing the term of the previous president and then be elected to a term of his own, or else leave office after serving two years of his second term.

Lyndon Johnson came the closest to serving 6 years - about 2 months over.

Coolidge served about 6 1/2 years. I believe all the rest who served more than 4 years served seven or more.