How far in advance does a subpoena (as a witness) need to be served in California?

Compelling witness testimony and records in criminal proceedings is generally governed by California Penal Code sections 1326-1332 and California Evidence Code section 1560(b). Witness indentity, and contact information is normally exchanged between counsel for the parties at a minimum of at least 30 days pre-trial so it is feasible that a subpoena may not get served on a witness until less than 30 days pre-trial. Courts will allow reasonable notice to the witness. If you are subpoenaed as a witness and you fail to appear on the date indicated or fail to agree to a different date by contacting counsel for the subpoenaing party, you can be held in contempt and possibly fined. Do not ignore the subpoena but contact the attorney for the subpoenaing party immediately. (either the D.A. or the defense attorney - whomever is seeking your testimony) You normally can't be compelled to travel over 150 miles to testify but a judge can order otherwise if compelling reasons exist requiring your testimony.

It just happens that I recently wrote a legal guide on this very issue. The link appears below. A subpoena can be issued at any time, including the day you are required to appear in court. While another answer says you are entitled to "reasonable notice," you are disobeying the subpoena if you do not show up in court as soon as possible. In addition to being fined if you fail to show up as ordered, the court can issue a "body attachment," which is essentially the same as an arrest warrant. It authorizes law enforcement to take you into custody and hold you until your testimony is needed. You are not obligated to contact the DA. Depending on whether or not you want to testify, that may not be the best thing for you to do.