I've done some analysis on
[Charred Twilight Scale], a semi-full ICC clear, around 140 procs for the trinket.
The minimum duration between procs was 50 seconds;
The average duration between procs on boss fights was 53.3seconds for a fire mage.
The data fits a model of 50s ICD and 30% proc chance on spellcast and makes the average uptime for a fight to be 28%.
EDIT: found a few more logs on WoL, I'm quite certain now that the ICD is infact 50s; for affliction warlock the average time to proc it seems to be around 55.6s.
EDIT2: yet another from shadow priest, 50s ICD 53seconds average time between procs;