How long do concussions normally last?

I was wondering how long do concussions normally last? Ive had a severe one for 4 months, with symptoms of severe headaches/migraines, pressure in my head, severe dizziness, nausea/vomiting, sensitivity to sound and noise, seeing stars and specks, even severe depression, (which came after my concussion, but also could also be caused by other issues in my life). I got an mri and nothing showed up. My doctors say it is extremely rare for this to be happening, and im being sent to a opthamologist, neurophycologist, therapist, concussion clinc, and regular doctor. Ive tried many medicines none work. I just want the pain to be over with. So i was just wondering are these symptoms normal to still be happening now, and how long do concussions usually last?