How long is it supposed to it take for MRI results to come back?

Depends. Did you have it at an outpatient office? A hospital? All scans should be read within 24 hours (and almost all in less than that). Where the report goes is then the problem. Did you ask for a copy of the report? If not it probably went to your doctor who ordered the test. You should contact that doctor's office to ask about the results.
When read. It depends on how quickly after the scan is performed to be read by a radiologist, how quickly after that the impression is transcribed or verbally communicated to the physician who ordered it, and then how long it takes him to have the time to contact the patient or see them back in the office.