Dead sims - any deadline for funeral?

Simmerville Unless something has changed with L&D, not in world ghosts get culled after about 90 days game time.  You can extend that if you have their urn or tombstone and are able to use the Strengthen the Bond option.  If you don't, they will get culled at the end of 90 days.  If you don't have the urn or tombstone and want to keep them in your game, they need to be playable and in a household in your Manage Households My Sims tab.  The household doesn't have to be to be on a lot so you can leave then as not in world, but they do need to remain playable.  Otherwise, they get culled in a vanilla game. 

So, if you make them playable, put them in a ghost only household and then move them out of a lot to not in world, they should hang around until you want to do a funeral.  I am not sure how funerals work for a played ghost though.  You might have to set them back to unplayed by moving them into a household on their own and then moving that household to the Manage Households Other tab first.  Tbh, I haven't gotten around to holding a funeral yet so Idk how they work so this is more or less my best guess. 

  Hope this helps.