Spawn time is not a straight up 13 hours. Window seems to be anywhere from 8 to 13 based on the times I've seen.
[Fri Jul 14 17:10:05 2023] Verina Tomb has been slain by Calalorne!
Think that it was a straight up 13 hours I logged in at 5:50 AM the next day and stayed through 7:10 AM and recorded no further spawn until the below
[Sat Jul 15 13:35:51 2023] Verina Tomb has been slain by Vexias!
[Sat Jul 15 23:27:05 2023] Verina Tomb has been slain by Fever!