There is a quest file
per mob, so just need to define a trigger for the mobs that you want depop.
Still, that makes several files, but you can symbolic link them to one using proper varaibles.
This behaviour could also be shared using the default file (used when no mob-specific file is found), where you would alter the EVEN_SPAWN() to setup a timer only for some npc ids, but this make a complex routine (even using global vars), as was pointed out
There is a problem, though : you can control depop time (you need to retriieve current time if possible), but not spawn time AFAIK (i.e. if the mob gets killed in the middle of the night, which happens sometimes
).