How long does an expungement take?

According to California Penal Code Section 1203.4, relief shall not be granted under this section unless the prosecuting attorney has been given 15 days' notice of the petition for relief. The probation officer shall notify the prosecuting attorney when a petition is filed, pursuant to this section.  It shall be presumed that the prosecuting attorney has received notice if proof of service is filed with the court. If, after receiving notice pursuant to subdivision (e), the prosecuting attorney fails to appear and object to a petition for dismissal, the prosecuting attorney may not move to set aside or otherwise appeal the grant of that petition.

This is of course after you have completed all other underlying conditions, which include but are not limited to completing probation, paying all fines and restitution and not being currently charged with a crime.