Assuming the Wizard wins initiative over the foes, that he gets 5 foes in an area, and he knows that Sleep is best used in this encounter:
With a 60% chance to hit, it hits 3 foes out of 5 on average.
Each of these save 55% of the time, so typically, the Wizard Slows 3 and makes 1 of these 3 unconcious (1.35 unconscious guys to be precise on average). That one comes out of unconsciousness typically within 2 more rounds.
You have to look at sleep in context of its most powerful use, which is with an orb wizard. With 16 wis, that's only a 40% chance for each person to save