August 2018 guidelines say about patients in vegetative states or unresponsive wakefulness syndrome, even after 3 or 12 months:
"a substantial minority ... will recover consciousness... While most of these patients will be left with severe disability... some will regain the ability to communicate reliably, perform self-care activities, and interact socially" (p.6)
They say doctors should stop talking about permanent states and tell people the "prognosis is not universally poor," though after 3 months for non-traumatic injuries (loss of oxygen to the brain) and 12 months for accident injuries (trauma), they should emphasize "the likelihood of permanent severe disability and the need for long-term assistive care" (p.4).
The guidelines also recommend transfer to a specialized center.
In 2018 a man was about to have his life support removed after 3 weeks in a diabetic coma, when his doctor father arranged to have him air-lifted to a specialized hospital, where he recovered.
This page gives a very brief summary. Families with someone in a coma need to read the guidelines, the studies they cite, and any newer studies, to find what is relevant to their situation. Most doctors and hospitals are not experts in this area, which is why the guidelines recommend transfer to a specialized setting, and why the doctor above did so for his son.
Researchers in Cambridge and Liege found that 5 of 54 patients in persistent vegetative states could respond to yes/no questions through MRI. "5 were able to willfully modulate their brain activity. In three of these patients, additional bedside testing revealed some sign of awareness, but in the other two patients, no voluntary behavior could be detected by means of clinical assessment. One patient was able to use our technique to answer yes or no to questions during functional MRI; however, it remained impossible to establish any form of communication at the bedside." They used functional MRI to measure responses. NEJM 2010, Monti et al.
Researchers in Paris found that 2 of 22 patients in persistent vegetative states could recognize patterns of sounds in ways that only conscious people can. "Interestingly, these 2 patients showed unequivocal clinical signs of consciousness within the 3 to 4 days following the experiments." The experiments measured responses with "high density scalp EEG," and they show a picture of the network of EEG sensors on a patient's head. Neurology, 2011, Faugeras et al.