Appears to be between 42-62.
root@ex2300c > show chassis temperature-thresholds Fan speed Yellow alarm Red alarm Fire Shutdown (degrees C) (degrees C) (degrees C) (degrees C) Item Normal High Normal Bad fan Normal Bad fan Normal FPC 0 CPU Sensor 42 62 65 65 75 75 80 FPC 0 PSU Sensor 40 61 65 65 75 75 80 {master:0} root@ex2300c>