According to the Riverside Municipal Code you have 10 calendar days from the issuance date to respond. A notice is sent out reminding you to pay or contest your citation and also indicates when your citation is due prior to becoming delinquent. Citations can be paid by:
Phone: (800) 418-4110
Online: http://www.pticket.com/Riverside/
Mail: City of Riverside, PO Box 3808, Tustin CA 92781-3808
Office: City Hall, 3900 Main Street, 4th Floor - Parking Services, Riverside, CA 92522