I posted the following on WoWhead:
For people looking at camping the spawns, I noted the following spawn times tonight:
8:52pm (ish, saw it up, but didn't see it actually spawn)
and then we'd run out of toons to kill things with. I suspect that the crystal follows the spawn times/pattern as any other rare - that is, 30-60 minutes after the last spawn. I don't know if this is timed from the spawn, the buff being gained by someone, or the buff wearing off (in which case, you may wind up a minute shorter than you'd expect if someone gets the buff then promptly dies).
I'll likely update this with more times when I spawn camp again.
Also: I hit the crystal at 8:52, and the corpses despawned by 8:56. If you're camping just to see when it pops, make sure you're checking back every 2-3 minutes so you don't miss the bodies!
I've also started recording the coins looted (counting kills is hard) vs scary sprite spawns (although I suspect it's random as I've seen them spawn basically when the crystal is hit)
83 coins 0 sprites
80 coints 0 sprites
123 coins 1 sprite
180 coins 1 sprite
I've killed another scary sprite before, but no pets yet