Each minute that defibrillator is delayed reduces the chance of survival of a cardiac arrest victim by how much?

Every minute counts. A rhythm which can be converted should be done as soon as possible. After 4minutes a good outcome is unlikely.
Not specific. Implantable defibrillators fire within 10 seconds, and are very successfull. I takes 5 minutes of cerebral hypoxia for brain death at 37centigrade. If heart fibrillating and no CPR done, brain death will ensue within 5 minutes. If first defibrillatory shock delivered within 30 seconds of arrest best chance.