When you place NPCs in the game which already have a health of zero do they stay in the game indefinitely until the player visits the cell they are in or do they disappear after a set amount of time? I have one NPC like this placed in a mod and want to ensure that they will still be there when the player arrives regardless of how long that may take from the start of the player's game.

Check the box to make the corpse permanent. Because that will stop the corpse from going away.

If you want to see how to do this, load up the Hlaalu Council House and there is a Perma Corpse in there to study.

Thanks, CCNA. I know I can check for their corpse to persist. I was just curious as to when, if ever corpses disappear on their own and whether that amount of time begins at the start of a new game or the first time a player enters the cell.

The cell has to be visited for the 72 hour countdown to begin.

I'm not 100% sure if the NPC needs to be interacted with or not, but I don't believe so. Entering the cell should be enough to start the clock.

If the Make Permanent box is checked, the corpse will never go away. The game can't remove it.

As for other non marked corpses, the counter starts when the cell is entered. But, this only affects corpses that are not marked permanent.

What would be an interesting experiment is to see if the mod would replace those unmarked corpses after 72 hours (after you reload the game once they are removed by the timer). Quite frankly, I don't see that happening, but I don't remember any cells that already had dead corpses in it when I entered, other than the perma corpses. So, it could be that the game will remove any corpse that is not marked permanent. Which is probably why they added that option to the NPCs.

You could do a quick experiment on the one in Hlaalu Council Hall. Make a mod that removes the check box, load up the game, visit the council house, then wait somewhere for 73 hours, then go back. It might take longer than that though. I would carry this out for 31 days before I called it good, or bad.

I am confident that the NPC's cell must be visited before there is any danger of the body being removed by the 72-hour effect. Certainly making the corpse persistent will prevent the body from ever being removed, but that usually not the reason for marking a corpse persistent.

Merely setting an NPC's health to zero in the construction set will result in the NPC displaying his or her death animation (complete with groans of pain). If the player is near the NPC when the cell is loaded the effect is unbecoming of someone supposed long expired. Marking Corpses Persist prevents the death animation and the NPC will be sprawled on the ground at the moment the cell is loaded. If you do not want an 'ever-corpse', the NPC can be disabled via script (or dialogue results) when appropriate (e.g. after quest is completed by checking a journal index).

@CCNA: I do not recall a body in the Hlaalu Council Manor. There is Ralen Hlaalo's corpse in Hlaalo Manor in Balmora. Perhaps that is the example you intended.