With 10mil subs still they will take a long time to fade, but if you look at that chart, the first significant dip in its history was post Wrath of the Lich Kin

With 10mil subs still they will take a long time to fade, but if you look at that chart, the first significant dip in its history was post Wrath of the Lich King, followed by the current dip which occurred after Cataclysm. They regained their subs after the wrath dip, but this one seems to be ~4x as big. It will be interesting to see what happens when Pandaria comes out, I doubt that they will ever return to their peak, but that is the point where we will _start_ to be able to make predictions on how long the demise will take.