I didn't do a lot of running away in D2:ED, but you can return before they have fully recovered. There may be some distance limit, where if you get too far away or teleport they will automatically heal, or something.
The only places I had to hit and run a few times were at level 2 with the group of goblins by the necromancer's cave (particularly the level 4 chief) and at level 7 taking out the group of level 8 skeletons at the front gate of Lovis' tower. With the skeletons I retreated down the hill to eat something and regenerate behind the tree, and did notice partially and fully healed skeletons when attacking again. The tree is just outside the skeleton's range, so I'm not sure if going farther would have made a different (over and above the extra time it would take to get back).
I don't remember if opponents in DD regenerated their hit points. The only opponent I recall having to leave was the orc war drummer on the trail south of Aleroth; he couldn't hurt me, but with the weapon I had at the time I couldn't do damage faster than he could heal himself.
