I'll speak of what I know.
Because my team was put together in very short order, I could not get everybody for a year. I could get some for a year and some for six months. It became very clear very quickly that we had to be there a year. For the team that replaced us, I think all but one are there for a year.
In Afghanistan, when you're dealing with Afghans, trust and relationships are everything. You have to build that trust up; you can't be disrupting it every six months. Every time I had to rotate one of my officers out of those places, I had to go and hold a lot of hands and make sure they didn't think we were abandoning them.
As far as the units down south are concerned, General Fraser is the brigade commander, for example, and I believe his tour is nine months anyway, as is that of some of his staff in that headquarters. For the rest, that's a force generation issue that others, like General Hillier or General Leslie, are far more capable of answering than I am.