How long does cns depression take to happen?

Depends: Hard to say. If we're just talking about the mental health issue of depression, some are more susceptible than other to vegetative states. Some have an agitated depression. If other medical issues are involved, it may depend on the progression of that illness. Parkinson's disease, als, cp, etc, result in CNS depression at the later stages of this disease.

Can be very quick: Central nervous system (CNS) depression means decreased neurological activity in the central nervous system, which is made up of the brain and spinal cord. It generally happens in response to drugs, medications, or alcohol. It can be very quick, depending on what is taken. By the way, this is different from depression of mood.