I think that Pat misunderstood your questions, and hence, my answers. You asked for the “usual” tour length in Iraq. It is twelve months. Here’s some supporting documentation: http://www.msnbc.msn.com/id/18059112/. It’s an online article. Your answer is in it’s first paragraph.
Second, you did not ask about military leave, but, rather, the length of time between tours. Currently, it is twelve months. This is not leave, it is just a non-combat assignment. Here is a NY Times article that answers both of these questions: http://www.nytimes.com/2008/04/06/washington/06military.html.