How long do you wait? A long time, probably.
Two to three months is not unusual. The problem is that there are too many people in the process, and too many steps in the process, and one day late can easily turn into one month.
So, if you tell them about the MS, which you have to do by law, they send you a form. You fill it in, return it, and it waits for the next meeting of the medical review panel. They write to your Neuro (or whoever you put down on the form). The Neuro writes a reply (whenever they spend a day on the paperwork) and returns it. The form waits for the next meeting of the medical review panel. They make a decision. You then get to hear quite quickly. But, the whole thing has taken time.
So I was driving for about a month without a licence. They tell you that their letter will serve as a licence. This is OK if you do not want a proper photo licence for some reason. It gets better - they ask for your old licence (still current) to be returned at an early stage in the process, and “you can still drive using their letter as a licence”. Yes, really! I cannot think of any US rental car outlet that would accept a letter in place of a licence. I can imagine just what a Gendarme would say if you were stopped for any reason in France. Of course, I did not return my licence. They never asked for it again. When I got the new one, the old licence went into the shredder.