NPC's will run out mana, but when their aggro list is cleared, ie they kill everyone or they all zone. Thier mana goes back to full really fast. Same as npc SK's regain thier harm touch soon as thier aggro list is cleared. careful with that memory blur :)
If npcs had infinite mana, then we've all been wasting our time mana seiving lady vox and all those other casters