Never? It's classed as an open-world zone, so for gameplay purposes it will remain intact indefinetely as it needs to be for players who have just reached that area to access. There's not going to be any major changes to it to refect G'raha Tia's comment in the ending that Bismarck's air bubble and the "lights of the phantom Amaurot" will fade in time.

Same with Vauthry's fake heaven and the giant Talos in the background of Kolhusia will eventually fall back to earth as well in time as the aetherial incantations placed on them wind down as time passes, but you'll never see it happen as it's too far in the future for the purposes of gameplay.