How many times do 75 percent of al Qaeda's top leaders need to be killed before the terror group is dead?

This is the interesting question posed by Bill Roggio over at The Long War Journal.

Although I think his skepticism regarding the limits of our metrics of success is warranted, I wish he would propose an alternative means of measuring progress against al-Qa'ida.  Until such a metric exists, political leaders don't have much choice but to use body counts from our decapitation campaign to signal at least operational (if not strategic) success.