I ordered the DC conversion kit from Adams Aviation at Biggin for my 13.4. Excellent! I wouldn't fly without it now, and the distraction and fatigue levels are much lower when you push the switch and it all goes quiet!
I don't know for definite the battery lasts (Duracell Ultra 9v), but think it is about 15-20 flying hours. Make sure you disconnect the headset from the battery box when you put it in your flying bag and then if the switch comes on it won't be trying to cancel the noise in your bag!