On PvE(-C) I constantly come across a bunch of thralls where a base used to be. Or some lone thrall or pet in a random location.
I would suggest they have the same decay time as buildings and also follow the rules of buildings when it comes to claim radius.
So if you leave a thrall/pet far away from a base with max decay, the thrall/pet will decay quite fast, just like any other single building piece.
On PvE(-C) there’s little I can do with a bunch of thralls just standing aimlessly out in the open, and I’m pretty sure this affects performance on both sides.