Answer:
(in the US) Nationwide statistics are difficult to come by, but it is not very often. If you raise the insanity plea as a defense against criminal prosecution, if that defense is successful, all it will mean is that you might be acquited. However, you will never be exonerated of your actions and the chances are overwhelming that the defendant adjudged criminally insane will be be institutionalized (sent to a psychiatric hospiital or the psychiatric ward of a prison) to serve at least a good portion of time undergoing psychiatric evaluation and treatment.