Sunni & Shiite Militias

Iraq has two major Muslim sects; Sunni and Shitte, it also has two major militas running the streets of Baghdad and other Iraqi cities. In between these two militias, currently engaged in an undeclared civil war are US troops. How long will the US be there? How long will the Sunnis and Shittes kill each other? What is the road to an independent Iraq? How long will it take and what will be the continuing role of the US military and its coalition partners?