Report Abuse. It all depends on how much Acetaminophen (Tylenol) you took. Overall it's half life is 2-4 hours, meaning if you took 1000 mg of Tylenol, in 2 to 4 hours, 500 mg is left, then after another 2 to 4 hours, 250 mg is left and so on... Source(s): learned in my pharmacology classes in medical school.