I've spent a lot of time in a rehabilitation facility and have seen many terribly impaired patients recover remarkably. But of course every situation is different so I couldn't give you a time period for a hypothetical.
But one good example might be Patricia Neal, the actress (she had a starring role in The Fountainhead). Neal suffered a series of strokes in the 1960s, after which she couldn't talk, couldn't walk.
Neal was profoundly depressed and didn't want to live. Her husband, writer Roald Dahl, practically had to force her into therapy (what a contrast with Michael Schiavo).
It took months for Neal to recover her speech and ability to walk. But she did. She recovered fully and was even able to resume her acting career.
Terri, in contrast to Neal in her initial depression, wants to live. How do I know?
Watch this video of Terri's response to a doctor's command. Stick with a good 30 seconds. You will see her will to live.