How often in real life does ventricular defibrillation work?

Time since arrest and cause of arrest are the main factors. But it's not just whether you induce a working heart beat. If you get the chemistry right, you can get a heart beat going in a lot of dead people. Doesn't mean they can survive or function again. What matter is that if the heart still has the capability to function and is still trying vigorously but is in V-fib, there’s a good chance of converting it and getting a reasonably good outcome. Note that, contrary to the movies, it does not restart a heart that has no electrical activity at all. That has to be done primarily with chemistry. In other words, you’ve to get it to fibrillate before you can defibrillate.

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Answer:
Many variables determine the outcome. Is the patient in a hospital? This increases the chance of a positive outcome. How long was the patient in V Fib/V Tach before the defib was performed. How much heart disease is there? What meds are the patient on. Does the have a AICD implanted? If the pt has a history of Vtach/Vfib and has an AICD which internally shocks the patient the postive outcome is very high.