According to this very well-researched and highly relevant piece from RAND, the mean length of an insurgency is ten years. Of course this figure is downwardly-biased because some insurgencies haven't ended (like FARC in Colombia, which has been going on for 47 years) and these are more likely to be at the upper end of the scale. Ignoring for a moment all the specifics about the Islamic State insurgency, what in general should we expect its lifespan to be?
The various insurgencies in Syria and Iraq over the last decade are a bit like the bands of the 1970s/80s New Wave scene in New York in that many of them do not have clear start and end points. (In other respects, the two things are quite distinct of course.) According to Wikipedia, the modern history of the IS begins with al-Zarqawi in 1999, but as an insurgency it didn't take off until 2003. In 2004 it affiliated with AQ to become AQ-I, then became ISI in 2006. Its incarnation as IS is of course less than a year old. So IS's insurgency is either less than a year old, or possibly eight years old, or eleven years old, or perhaps older. What difference do these interpretations make to the expected time the insurgency has left?
The answer is 'not much'. Looking at the dataset RAND used, which consists of structured data and judgements about 89 insurgencies beginning with the Chinese Civil War, the decay of insurgencies is fairly constant over time. Unlike with, say, humans, knowing how long an insurgency has been going on for actually gives you very little information (by itself) about how long it has left. For at least the first twenty years of an insurgency, the expected future lifespan remains at between 9-13 years. The base-rate for an insurgency ending in any given year is only around 5-10%.
All things being equal, then, we shouldn't expect the IS insurgency to finish sooner rather than later. Of course, not all things are equal. RAND's analysis points to a number of factors that promote earlier resolution of insurgencies, including withdrawal of government support, desertion, infiltration, intelligence penetration, and - perhaps significantly - use of terrorism, which they found often works against the perpetrators' chance of victory. However, they also found relevant factors that lengthen insurgencies, including a weak government, and multiple parties involved.
What all this points to is a lot of uncertainty. We should be suspicious of anyone claiming to have a very good idea about the prospects for IS, Iraq and the region in general.