Probably not high. The bastards know the terrain far better than we do. Short of bombing them to smithereens, they are GOING to evade pretty much everything we try to do. I mean, they may be assholes deserving of the absolute worst punishment (ie, being turned into melted slag), but they're NOT stupid.
Again, we need to learn some lessons from Hiroshima and Nagasaki. Force them to surrender by simply bombing them out of existence until they do what the fuck they are told, when they are told, how they are told. Until the day comes that we say, "Jump!" and ISIS's only question is "How high?", they should be subject to continual assault. But one nuclear strike on their main base should do it. It worked with the Empire of Japan, it also ought to work with the so-called "Caliphate of the Islamic State of Iraq and the Levant" (ISIL). Ok, it took two strikes on Japan, but close enough.
While we are at it, it might be a good time to spread pork grease on Mecca and Medina if any other Muslims give us any grief, and advise them that the next step will be to bomb both cities into pulverised gravel if there is any more disobedience. Time for them to see how real wars are fought and won.