My MS MD said that the normal course at MY clinic is. ...

one every six months for the first year, then once a year for 5 years, and if no sx or problems remain they only ask for once every 3 years.

Since I have a glioma I get scanned every six months. I had to be different.

If you are having issues, they may wish to see if your disease has progressed. They can tell if you have new lesions, and if they are active.