It all depends on the enemy, and what they think of the attack they just endured. I've had some humanoid tribes call back hunting parties, set additional traps and alarms, and set ambushes...strategies to specifically counter tactics that survivors reported the PC's used. Some will team up with neighbors (the old Keep on the Borderlands module even outlined such contingency plans, and made use of an ogre mercenary who occasionally teamed up with the goblins living nearby). I've also had smaller humanoid groups completely bug out...with their loot. Less intelligent creatures may just like their wounds and try to recover. More intelligent groups...or those that can figure out where the PC's went, could launch counter attacks. Just figure out the intelligence, resources, and organizational ability of your villains, and...keeping with what works best for your campaign, let them take the appropriate action. Will they build traps, alarms, ambushes? Do they have additional manpower in the field? Can they hire mercenaries, assassins, magical assistance? Do they have healers? Will their casters shift their spell lists away from "daily use" combinations to "pure combat" lists?