Legal Question in Civil Litigation in California

It depends. As soon as possible. There are other attacks on regulations that come to mind, for example that the regulation violated the state Administrative Procedures Act or other state or federal law.

Rule #1 of litigation, never bet on any legal opinion of when the statute of limitations will run out, unless you think it has and need to decide whether to proceed. Otherwise, just file as soon as possible. With that said, since unconstitutionality is either a defense, or a basis for mandamus when the government refuses to act, I'm pretty sure there is no statute of limitations until the government acts to enforce or refuses to do an act. You must have enforcement of the law or regulation by government conduct, which you can, of course, induce so as to give grounds to sue. For example, the plaintiffs in the same-sex marriage cases all had to go down and try to get a marriage license and get turned away before they could file the test cases. Larry Flint had to be charged with pornography before he could challenge the pornography laws as unconstitutional in his defense. The courts will not hear hypothetical cases or abstract challenges to the constitutionality of a law or regulation.