Hardly ever. Different sources will give varying numbers, but generally you'll hear that it's only plead in about 1% of all criminal cases, and only passes in about a quarter of those.
about 26% of all court cases
When someone successfully uses the insanity plea then they would plead not guilty by reason of insanity
An insanity plea -- the contention that the individual could not distinguish between right and wrong at the time of the crime -- can be used in any criminal proceeding. It is, however, more effective in some than in others.
The actual plea would have to be "Not Guilty by Reason of Insanity." The job for the defense attorney would have to be to prove the claim of "derealization" as sufficient enough to qualify under that plea.
The state of Germany
Some successful criminal defenses are used by the defense attorney to the accused and some defenses include: insanity, temporary insanity, and the non-guilty plea.
People can no longer plea insanity due to the Affordable health care act.
Fred'k H. Teese has written: 'Insanity as a reply to the plea of suicide'
sanity and insanity are definitions used in the legal system
(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.
As far as I can tell there has been no definitive answer in the Holmes case. The judge accepted his plea of insanity in June 2013. There was word that his defense was thinking of withdrawing that plea, which would indicate that his legal council probably needs psychiatric care also. Holmes best defense would be insanity. He would then escape the death penalty and probably would serve his sentence in a mental hospital as opposed to prison.
There are several differences but the main difference is a successful plea of insanity will, in most states, result in a verdict of "not guilty" and commission of the defendant to a mental institution. Diminished capacity merely results in the defendant being convicted of a lesser offense.