If the load as seen by the diodes is inductive or resistive then the diodes conduct for the complete half cycle. If the load has part capacitive load then the angle of conduction depends on the amount of ripple on the output. The lower the ripple the smaller the angle of conduction. Draw out your waveform, put a line across at peak voltage and another at peak volts- ripple volts. Look at the area of your waveform between theses lines, this is the conduction angle. Remember, peak volts = transformer volts - volt drop across diode.
Frank