The timer starts when the victim arrests. Period. It doesn't start when you call 911, it doesn't start when the ambulance gets there. I starts when the victim dies.
Please don't ever wait for the ambulance to get there. The result is overwhelmingly going to be bad if you wait.
I'm not talking about your seeing some random, sketchy stranger go down. The far more likely scenario is that when you see an out-of-hospital sudden cardiac arrest, it will be a family member, a friend, or an acquaintance. Learn what to do. Better yet, learn what to do and get a personal use AED because two thirds of all cardiac arrests happen in the home, and immediate CPR + prompt defibrillation can change the average home survival rate from 2% to 75%..
If you cannot take an AHA or an ARC course right away, go to www.slicc,org, click on the For Past Trainees link in the left column, and download the class video and watch it. - and then take the AHA or ARC course when you can.
Do it now, PLEASE!