How many times is a person generally shocked or defibrillated before the doctors determine it isn't working?

As many times: There is no right or wrong amount of shocks here -- since this is desperate times and it would be completely the call of the managing physician at the time -.

Variable: There is no rule. Every case is different. Factors that are considered include: age, underlying disease, overall prognosis, premorbid quality of life, length of time CPR was done before the first defibrillation, rhythm following defibrillation, reversible factors e.g. Drug effect or electrolyte imbalance. Whether the person was hypothermic? The setting e.g., hospital or airport.