Yeah, I think you just need to be direct with them and tell them that it's a myth that concussions heal up in a few weeks. You can say that it's just not well-known that it can take months or years to recover from a concussion.

Another thing I would suggest is to use the term 'Traumatic Brain Injury' rather than 'concussion'. I think when people hear 'concussion' they think of something mild and not-so-serious; when they hear 'traumatic brain injury', they think of something much more serious. Even if you just call it a 'brain injury', that conveys the same idea. I think once I switched terminology and calling it a brain injury, people started to get it.

Of course, some people just won't get it, and there's nothing we can do about it. They're ignorant, unsympathetic, and self-absorbed, and they're going to stay that way. The best we can do is to try to not let those kinds of people bother us.