Is it true every cigarette you smoke 12 min. subtracts from your life?

There's no exact number of minutes. I've heard anywhere from 5 to 12, but the most common number is 7.
We all know that smokers, on average, live shorter lives than non-smokers.
As for the smokers, if you were to average in how many years a group of otherwise healthy people smoked and how many cigarettes per day, then compare their lifespans to one another, you would find that the ones who smoked less would live longer on average, but not by much. That's because there is absolutely no way to quantify how many minutes a cigarette takes off a person's life- there are simply too many variables. Aside from environment, which is a big enough problem, there's also genetic variability to contend with. Some smokers who smoke the same amount for the same number of years develop emphysema or lung cancer and some never have any complications at all and live to be just as old and healthy as anyone else.
So no, there is no exact number for everyone. Every person who smokes has their own.