Short Answer:  It's a legal determination ultimately left up to a court, but takes into account a variety of factors.  Typically, the most controlling or popular factor is when the property is ready for occupancy.

Long Answer: This is the hardest question to answer in California's mechanics lien law. Unlike the majority of states, a mechanics lien must be filed in California within a certain period of time from the completion of the entire project and not simply conclusion of the claimant's work.  Accordingly, a party that furnishes at the very beginning of a job (i.e. the foundation or site work folks) may have a ton of time to file their lien, while the finishing contractors may have a tiny window.