Item Drops and Finds Version 2 - Post BB

To add to this, I think you can predict how soon the boss will spawn from the message you get from defeating eliminating monsters.

"I sense something in the darkness" - Not sure if these are the exact words but when this message pops up it's usually about 2-3 elite monsters before the boss with the bonuses spawn.

"I feel something in the dark energy" means five or less.
"something something makes the place grim" - haven't done enough testing but I think I've counted 5~10 before the "I feel something in the dark energy" message starts showing up.