hi welcome to healthcaremagic
it depends on many criteria like cause, location, severity and extent of neurological damage. it usually takes days to years depending on those factors
Coma may result from a variety of conditions, including intoxication (such as illicit drug abuse, overdose or misuse of over the counter medications, prescribed medication, or controlled substances), metabolic abnormalities, central nervous system
diseases, acute neurologic injuries such as strokes or herniations, hypoxia, hypothermia
or traumatic injuries such as head trauma
caused by falls or road traffic accidents.