When do the Tier 17 world bosses respawn?
Solution 1:
Drov and Tarlna respawn quickly - "about 15 minutes" seems to be the most quoted respawn time.
However, only one of those two bosses will be active at any time! On days that Drov is respawning, Tarlna cannot be found, and vice versa.
Tip: Check whether LFR shows an existing raid for one of those bosses and, if so, join that raid.