Likelihood of fatal fibrillation?

It Varies. Presumably you are asking about ventricular fibrillation, since atrial fibrillation would hardly ever by fatal. The risk of vf, or cardiac arrest, varies widely depending on what underlying heart conditions exist. Cardiac arrest occurs to about 400, 000 americans annually. The risk in some serious heart conditions may be 20-40% a year. For most people, it is much less than 1% a year.