Legal Question in Civil Litigation in California

As soon as possible. If your process server serves it very soon before trial, and you need the court's assistance to enforce it, you will have to tell the judge why it was necessary to wait until the last minute. You might have to pay sanctions if you inconvenience a witness unnecessarily.

There is no set time for serving subpoenas prior to trial. You really should have them served as soon as possible, in all fairness to the witness that you want to attend the trial.