Taken from WoWhead:
LFR: 1x Eternal Ornament per wing. 3x per full clear.
Normal: 1x Eternal Ornament per boss. 8x per full clear.
Heroic: 10x Eternal Ornament per boss. 80x per full clear.
Mythic: 20x Eternal Ornament per boss. 160x per full clear.
So if you’re only running LFR raids, forget making yourself anything for about 80 weeks. Normal takes it down to about 30 weeks.
The heroic and Mythic raiders are set with this. 3 weeks for heroic, 2 for mythic. Of course that’s assuming your party let’s you teleport out to complete the second phase of the quest chain. If they don’t it’s still pretty quick.