Technical answer: In California, the law requires the court to have jurisdiction over both parties for more than six months before the marital status changes, which is different from a divorce being final. To understand that in real-life terms, you need to know what “jurisdiction” is, and what it means for a judgment to be “final.”
Jurisdiction: When you sign and file a Petition to end your marriage, you give the court “jurisdiction,” that is, the judge has power over you to make orders you have to follow. When your spouse is served with the papers, it gives the court “jurisdiction” over him or her.
Final Judgment: To finish up a case, a judge has to sign the final decision (a judgment), and the clerk has to file it. When that’s done, and there is nothing left to do with it, the judgment is “final,” (meaning a clock starts running for appeal, a request for a new trial, or other things). That happens fairly quickly if everything is agreed. It would be possible to do it in as little as five weeks or so, but the court usually doesn’t work that fast – three months is more realistic.
Putting it together: If you are on a fast track (finishing in less than six months), find the date the Respondent was served, and count six months and a day from then. For example, if you file a Petition on January 15 and serve the other party on January 20, six months and a day later is July 21. You might get a judgment signed by a judge and entered on March 30, but that judgment will have a section in it that says when the marital status ends (when you stop being married). In that case, the judgment would be final on March 30, but it would say you are single as of July 21. Get married before that, and you have committed bigamy! If the other party was served more than six months before the judgment was filed, then you’re single when it’s filed.