What is the test for a defendant pleading not guilty by reason of insanity?

"When a criminal defendant pleads not guilty by reason of insanity, the finder of fact must determine by a preponderance of the evidence whether the defendant was insane at the time of the offense. ( 1026, subd. (a) ; [citation].) ... If the defendant succeeds in proving his insanity at the time of the offense, commitment follows unless the court determines that the defendant has fully recovered his sanity. ( 1026, subds. (a), (b).)" (People v. Tilbury (1991) 54 Cal.3d 56, 63, 284 Cal.Rptr. 288, 813 P.2d 1318.)