It depends on whether the autopsy was performed by a medical examiner, coroner, or hospital. It then depends on the turn-around times of the labs. And like the guy above mentioned, blood work and chemical tests on body fluids can take a long time, especially if they are sent to a lab that has a high volume of work.
It will also depend on whether the autopsy was performed for a case that will involve forensics and the law.
If the autopsy was performed at a hospital, the pathologist is required to sign the case out within a month. Unfortunately if they are busy, it may take the full month to get results. If the autopsy was performed at a hospital however, and you are the next of kin, you can call and request to speak with the pathologist regarding the case. The same is true for the medical examiners, as long as a legal case is not pending.
If you are not the next of kin, you do not have access to the autopsy results and will have to wait for someone else to tell you.
Hospital autopsy results usually include to cause of death and a summary of findings of the organ systems.
Medical examiner results will include a cause of death and any other relevant findings.
Work in a path lab, have been to the medical examiners on occasion, have helped with many autopsies