During an interview at the Defense One conference, Joint Chiefs of Staff Chairman General Martin Dempsey estimated that the US war against ISIS in Iraq and Syria will last "up to four years."
What he's basing that on is totally unclear, as the Obama Administration continues to escalate the war, and the scope and goals of the conflict at this point remain totally unclear, and growing all the time.
Gen. Dempsey himself has been recently pushing for major increases in the role of US ground troops in the war, suggesting it was time to start considering using them in offensive operations, particularly the retaking of Mosul.
Which is itself not even being attempted yet. The Pentagon is envisioning starting the counter-offensive against ISIS in a big way in Iraq in later 2015, and hasn't begun the 12+ month process of trying to create a fighting force for inside Syria, which means that won't begin in earnest until early 2016 at best.