Welf. & Inst. Code, § 631

(a) Except as provided in subdivision (b), whenever a minor is taken into custody by a peace officer or probation officer, except when the minor willfully misrepresents himself or herself as 18 or more years of age, the minor shall be released within 48 hours after having been taken into custody, excluding nonjudicial days, unless within that period of time a petition to declare the minor a ward has been filed pursuant to this chapter or a criminal complaint against the minor has been filed in a court of competent jurisdiction.

(b) Except when the minor represents himself or herself as 18 or more years of age, whenever a minor is taken into custody by a peace officer or probation officer without a warrant on the belief that the minor has committed a misdemeanor that does not involve violence, the threat of violence, or possession or use of a weapon, and if the minor is not currently on probation or parole, the minor shall be released within 48 hours after having been taken into custody , excluding nonjudicial days, unless a petition has been filed to declare the minor to be a ward of the court and the minor has been ordered detained by a judge or referee of the juvenile court pursuant to Section 635. In all cases involving the detention of a minor pursuant to this subdivision, any decision to detain the minor more than 24 hours shall be subject to written review and approval by a probation officer who is a supervisor as soon as possible after it is known that the minor will be detained more than 24 hours. However, if the initial decision to detain the minor more than 24 hours is made by a probation officer who is a supervisor, the decision shall not be subject to review and approval.

(c) Whenever a minor who has been held in custody for more than 24 hours by the probation officer is subsequently released and no petition is filed, the probation officer shall prepare a written explanation of why the minor was held in custody for more than 24 hours. The written explanation shall be prepared within 72 hours after the minor is released from custody and filed in the record of the case. A copy of the written explanation shall be sent to the parents, guardian, or other person having care or custody of the minor.