Oh, I see, sorry.

An area will respawn after you've travelled three areas away from it, I believe; it's referred to as zoning. The trick is, to find a looping path through four or more areas, and just keep moving in an endless cycle. You should never run out of enemies to kill that way.
Hm, I've tried that method but if I do that, I'll find sightly fewer enemies. But if I load the game save instead, I've seen a bigger number of enemies. o.O