Mostly, individuals rotated every 12 months. The 4th Infantry Division was in country from 66 to 70, the Americal Division (24th) from 67 to 71, and the 101st from 67 to 72. Beginning in 1965, the Army began a policy to insure no more than 25 percent of a unit's strength would be rotated in any one month. Managing replacements was not easy, certainly, but the rotations did not stop and restart the war each time someone rotated home.
Also, remember this difference: Vietnam was packed with draftees who were only available for two years. In Iraq, and elsewhere, we're not saddled with that problem.
I saw three complete rotations of unit sin 19 months, and I became institutional memory and if you know me that is a scary thought.