Depends on how picky one wants to be with all npc respawn timers. Running a simple query can take all npc's down to the standard "640" respawn time. (or less) (3600 = One hour)
Code:
UPDATE spawn2 SET respawntime = 640 WHERE respawntime > 640;
You can also set their chance of spawning to 100 %
Code:
UPDATE spawnentry SET chance = 100 WHERE chance < 100;
This would apply to
ALL NPC's on the server. But there is also PH's to consider as well, they would be affected. Also, some unique spawns may be controlled by a quest script.