Well. I know that Zugurosh had a spawn time of 10 minutes before the respawn rate change.
I never killed Zugurosh after that but you can check if it really is 5 minutes now or if it still is 10 minutes since the change didn't affect all monsters but only some.
It should really be the same for the rest of the Inquisition bosses.
When I was farming pharaoh bosses I was farming Mahrdis during the old 10x Improved Respawn (when Tibia was 1x respawn).
It respawned every minute so it did have 10 minutes respawn time aswell.
It possibly has not changed since then but if you want to find out and you cannot find the answer anywhere then you can check it out for yourself.