What i know from the time i farmed, skarr and the 2 other bosses in the pit of DiM, it spawns 8 hours after it was killed last.

But they might have changede it.

Skarr is the easiest of the 3, atleast form me as feral tank, Razza need more consentration as do the 3. boss mushgog or what his name.

about 50 boss kills and 6 orbs