(L&D) Graves, how quickly do they fade?

Thanks both! I'm not intending to keep all ghosts, it's just that there are soooo many popups warning me that spirits are fading, and not all are graves on that played lot or related to the played family either. Because it says only the first name it is often hard to understand where the grave is located, and the reason for my Q is basically that I had no understanding of how urgent it is once that popup shows.

I want to keep some ghosts, but I will never play them as a household member. I always build background history, so I want to keep just a few older that relates to my created history. Maybe I'll keep only one old ghost per family. Maybe it's easier to keep them going if they move to the cemetery lot, because the game spawning "Mourners" might help keeping the ghosts from fading?

Another thing is that I find no way to see how much faded a given spirit is. I assume it is based on the sum of relation levels, somehow, but I do not know if it helps say lighting a candle at the grave every night (on a home lot). No idea whether it must be done by someone with a relation to that ghost.

But basically, it would be nice knowing if "fading spirit" means that the ghost will leave shortly, or if the popup is mostly a reminder. If it all is restricted to 3 days and not relations, I think I should simply let go of all ghosts as I play rotationally mainly focusing on other aspects than ghosts.