How many long do you have to in college be a autopsy?

There's no such thing as being an "autopsy" in college. To become a forensic pathologist (the doctor who performs autopsies), you need to complete a four-year undergraduate degree, then four years of medical school, followed by a four-year residency in pathology and then a fellowship in forensic pathology (usually 1-2 years). That's a total of 12-13 years of education and training after high school.