Enter Your Body Weight (in pounds): = 12 oz cans of sweet, yellow, caffeinated goodness it would take to kill you
Coupla notes here: First, this is hardly scientific. It's based on the LD_50 of caffeine which is the amount of caffeine it would take to kill 1/2 the people in a given group if they were administered this dosage.
It's not based on a period of time, it's based on a concept similar to blood alcohol content, the amount in your blood would diminish over time.