It's more than 1 hour. Today I've found him, waited for more than 1 hour before killing him and he was still there. Atleast 2 hours, but I think he works like Arthom the Hunter and Oodok Witchmaster, they were all implemented in the same update in 2015. Both Arthom and Oodok stays in the respawn until the server save if they are not killed. For Tzumrah, I didn't tested it myself, but I also think he works this way because he has a 20-day cooldown.