It would be when everyone else had rejoined to the Force and the material universe had achieved thermodynamic equilibrium. Any souls left behind in an incorporeal existence, neither alive nor dead, would be stuck in eternal nothingness. The Mind Walkers were conned into precisely that. A false netherworld that neither brings immortality nor ascension to the afterlife. Likewise, the souls of the Kathol hadn't become one with the Force, they simply imprisoned themselves forever inside the Life Well. That's not ghosting in the sense Obi-Wan or Yoda did it. It's trying to remain in the mortal realm beyond the death of the universe.