I've killed this guy ~20 times during this year's Hardcore Heritage. Each time it dropped one piece of Glowing Reis armor and between 0-2 pieces of other loot on his loot table. And yes, it seems to be on its own timer, but a guaranteed pop when that timer is up.
Cleared entire camp several times (approx 4-6 ice giants plus a roamer icy terror or 2). Within 35-40 mins first Guardian of Vox spawned..roaming from his varied spawn points in northern open area..usually after there is at least one other ice giant respawned. Sees invis and drops 2-3 items ( one being Glowing Reis). Spin stun as always. Had him spawn 3 times in under 2 hrs..so im guessin 35-45 min spawns on the avg...based on 2-4 clears of area. Good Luck!!!
Camped this mob a few times. Always followed a 2 hour +/- 20 minute spawn cycle. Until one evening he spawned after 1 hour of waiting. No idea why, hasn't happened again since. The only thing going on at the time was 3 different guilds starting the Vox raid. (9 kills and no aug still, bah)