Day 2: One news channel last night claimed that the clearing of the city could take up to six months. That seemed excessive.
Basically, with ISIL being outnumbered and not having any real armored forces or artillery, they really have no way to contest the ground outside of Mosul. Like most of these other operations, I suspect that getting to and isolating the city won’t take that long (maybe a week or two). I also suspect that the majority of the ISIL fighters will withdraw and they will leave a holding force in Mosul. This is how these fights have happened several times before. Then there is a city-clearing operation, which is basically a mop-up operation. This could drag on for a while, depending on how aggressively it is pushed. The Second Battle of Fallujah in late 2004, primarily done by U.S. Army and Marines, took about six weeks. The Second Battle of Tikrit in 2015 also took around six weeks. The Battle of Ramadi in 2015-2016 took over ten weeks. The latest Battle of Fallujah in 2016 lasted about five weeks. Granted Mosul is bigger, but I can’t envision it will be that different. How many fighters is ISIL going to sacrifice to a lost cause?
Anyhow, article de jure on the war: Mosul offensive