Hard question to answer. There's more variables to this question than just the time - it also depends on how much charcoal you use. Normally you'd only need 2-3 pounds of charcoal in an enclosed space such as a car - and then if you waited 10-15 minutes until you got into the car, that would be good enough for you to go unconscious in about 10-15 minutes maybe? Now let's say you doubled that, and used 5/6 pounds of charcoal. That's gonna be producing twice as much CO. And if you wait two times as long, let's say 30-40 minutes. That's going to be a massive amount of CO in the car.
In my opinion, if you used 6 pounds of charcoal, and wait 40 minutes before you get into the car, that's going to put you unconscious QUICK. It's hard to quantify - but if you use my measurements, I believe you'll be out roughly somewhere between 2 to 6 minutes.
The charcoal method is an excellent method. You go unconscious so quick, and your brain never panics. Completely painless. Good luck!