California law does not provide a definition for what constitutes "full time" employment. California leaves this definition to each individual employer. Unless there is an unlawful purpose or discrimination, an employer is entitled to reduce the number of hours. There is no minimum number of hours.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney.