In Iraq and Afghanistan, the length of time you'll spend there depends on which service you are in and, to a lesser extent, what you job is. Most of the troops over there are army, and they stay for twelve months. But navy and marine personnel are there for six or seven months. That's because the U.S. Navy (which transports the marines), deploys its ships overseas for six months at a time. The ships can stay out longer, and often do when there is an emergency, but this causes more damage to the ships, and the morale of the sailors and marines. The U.S. Air Force sends it people over for four months at a time, because this long a tour has the least negative impact on morale. The army prefers the longer tour because when the troops first arrive, they have to spend more time and energy getting up to speed on what's going on, and what their predecessors were doing. The marines are not there to do as much civil affairs work as the army, but to just fight. So they don't have to deal with "getting acquainted" as much. All services have some people, usually those on high level staffs or senior commanders, who do a twelve month tour. Makes for better continuity and all that. This time around, the impact of the different tour lengths is being studied more intensively, and some long term changes may come out of all that.