It depends on the circumstances.
There is no such thing as a 'tour of duty' except as a public relations exercise.
Service people know this, only civilians would ask.
Afghanistan
The length of a tour in the Army is up to 12 months. There is a possibilty of going for only a month, but that's not classified as a deployment, it's a TDY(Temporary Duty Assignment.) Most units do not do TDY's to Afghanistan, but some units so, such as the Army unit out of Naples, Italy.
two years
7 months
Tour of Duty - 1987 Road to Long Binh 3-15 was released on: USA: 10 February 1990
"For the duration of the war".
The term used for a period of duty in a specific location or assignment is a "tour" (tour of duty).
Andy McNab's Tour of Duty was created in 2008.
The duration of Tour of Duty - TV series - is 3600.0 seconds.
The duration of Andy McNab's Tour of Duty is 3600.0 seconds.
Tour of Duty - TV series - was created on 1987-09-24.
Tour of Duty - TV series - ended on 1990-04-28.