I have been working on this and have something, but not 100% figured out. Delve bosses generally have a 5 minute spawn timer. Sometimes they will spawn more or less rapidly, but usually right on 5 minutes. I think if someone comes through thay doesn't have the boss achievement yet, it will initiate a respawning immediately. The loot timer for delve bosses seems to be the same, 5 minutes.

In public dungeons, it's a little different. It's possible sometimes to kill 2 bosses within a minute and get 2 drops, but after that there is a window you have to wait through. I think to ensure that you get a loot drop every time, the optimal time to wait is 2:30. Bosses spawn a bit faster than 5 minutes in pub dungeons also. Not exactly sure of the interval.