The minimum yellow light duration is regulated to 3s by the Federal Manual of Uniform Traffic Control Devices. In CA, the timing is additionally required to be

The minimum yellow light duration is regulated to 3s by the Federal Manual of Uniform Traffic Control Devices. In CA, the timing is additionally required to be no shorter than the Caltrans Traffic Manual, which sets a higher minimum time depending on the speed of the road. If you get a ticket and the yellow is shorter than that, you have a very good case for getting it thrown out.