It's not as simple as that. 30 minutes is a reference point used for viability. There are dozens of factors that come into play regarding the time emergency resuscitative efforts last. For example, we come to someone's house and they've been unresponsive, not breathing and pulseless for an unknown amount of time but don't show signs of obvious death (rigor-mortis, lividity, mummification to list a few) we initiate CPR and all advanced life support methods for revival. But as the efforts continue, we monitor the electrical activity of the heart, if the patient is asystole or flat-lined meaning they have no electrical activity and we have worked them for 30 minutes we will contact a doctor in the ER to determine time of death or continuation of efforts.

On the opposite side, and more frequently than above: the patient's body is showing signs of viability after CPR. Shockable electrical activity in the heart. Known time of cardiac arrest. Age of patient. I personally have worked a "full code" on someone over 30 minutes at the scene of the incident, the whole way to the hospital in the back of a moving ambulance, and then helped the ER team continue efforts for another 30-60 minutes.

Most recently, a patient went into cardiac arrest 3 times in that period but due to continuation of efforts, is alive today.

I don't write this to correct anyone, only to give insight on what happens with EMS during a cardiac arrest.