Penal Code § 915

When the grand jury has been impaneled, sworn, and charged, it shall retire to a private room, except when operating under a finding pursuant to Section 939.1, and inquire into the offenses and matters of civil concern cognizable by it. On the completion of the business before the grand jury or expiration of the term of prescribed service of one or more grand jurors, the court shall discharge it or the affected individual jurors.