Depends on how much of an effect and how many troops each of his planets has. Every planet you glass causes more unhappiness, but every troop he has on each planet causes some happiness to balance it out. There are other factors as well, but large numbers of police troops tend to dominate the equation when they're used.
Look in Happiness.txt for the exact effects of each event. Most people (and AIs) take Peaceful, but without knowing exactly what the enemy has, it becomes trickier to say exactly how many you need to glass.