Normally, 90-120 minutes after closing all the vents on a WSM, the coals should extinguish from lack of oxygen. If not, you may have enough of a leak around the door, or an out-of-round condition on the center section allowing too much air in that you may want to attend to. In any case, you never want to consider coals so-extinguished completely out, and treat them as such-- sort of like "there's no such thing as an unloaded gun".