Friend of mine was in a cycle accident at speed was in a “trauma” coma for about six weeks is alive but suffered some brain damage, the brain is very plastic though and was able to re-learn some skills lost from the trauma.

I think the time before it’s likely they won’t ever awaken is very much dependent on the type of injury. The level of brain activity can be monitored in more detail these days and the amount of “activity” and whether it is increasing or decreasing plays a big part.