The longest amount of time between a patient falling into a coma and subsequently regaining consciousness is 19 years, according to Everyday Health. Terry Wallis was rendered comatose after being paralyzed in a car accident in 1984. After spending nearly two decades in a coma, he awoke again on June 11, 2003.
There are different levels of coma, but in general, comatose patients cannot communicate, cannot move in a purposeful way or follow commands, and cannot sense or respond to their own needs or environment, as noted by MSKTC. In this state, eyes may open from time to time, but without apparent awareness.
Within a year of his accident, Terry Wallis' coma progressed to what is called a "minimally conscious state," but without any further improvements to his health, his doctors believed he would never recover. That he awoke at all was a major accomplishment, but, even more impressive, he had also retained many of his memories from before the accident, along with his language skills. Both of which can often be difficult to recover after a brain injury, such as the one Willis suffered.