If a man aged 76 yrs has cardiac arrest and goes in coma how can his brain be revived and in how many days?

There is no sure way to reverse a coma. If his electrolyte levels go haywire that is an indication that he is brain dead and that there is no further hope. Otherwise all you can do is hope and pray. Some people claim that talking to the person in a coma is helpful too.

I don't know what you mean by "revived". There is no way of knowing how long someone will be in a coma. The docs will most likely do a CT scan and an EEG on the brain to determine brain damage and function. If the brain was deprived of oxygen for too long, one of two things will happen. Either a brain damage or vegetative state or brain death. At brain death, the only thing keeping someone alive is the machines. Doubtful it would ever be days. Therapy for brain damage lasts months or even years depending on the severity. Best wishes.

The cruel reality of resusitation is that after about 6 minutes of down time the patients brain is either dead or dying.

If you call an ambulance , it can take 5-15 minutes to show up.

I'd ask your doctor for a realistic prognosis.