I have no idea how long his respawn is, but I know the 2021 patch notes said "Venril Sathir's remains will spawn every 20 hours instead of just over 26 hours." This is not true as written. Maybe they meant 20 hours after Venril Sathir is killed? I don't know. But I do know that I personally witnessed someone doing the turn in to spawn the triggered version of VS at 8:46pm, if it spawns every 20 hours, then the next spawn should be 4:46pm the next day. He did not spawn around that time. I stayed logged in a feigned so I would be able to check logs and see if he spawned early, if someone else came and killed him, but nobody did. It has now been 21 hours and he has not spawned. So the patch notes are for sure incorrect.
The normal version of Venril Sathir was killed at 1:02pm, so if VSR spawns 20 hours after VS is killed, I'll check again at 9am, but he definitely does not spawn "every 20 hours".