I've always been skeptical about the 3s ICD of Bone Shield; it just doesn't seem to hold true. So I went off to the Valley of Lost Heroes to test it. The idea was to get as many mobs attacking me at once as possible, ensuring hits by turning my back to them. Sadly, the combat log doesn't seem to log the loss of individual bone shield charges, but it does log the cast and total removal, of course:
2/7 09:19:48.367 SPELL_AURA_APPLIED,0x0100000002CBB83B,"Valen",0x51 1,0x0100000002CBB83B,"Valen",0x511,49222,"Bone Shield",0x8,BUFF
2/7 09:19:48.497 SPELL_CAST_SUCCESS,0x0100000002CBB83B,"Valen",0x51 1,0x0000000000000000,nil,0x80000000,49222,"Bone Shield",0x8
2/7 09:19:55.506 SPELL_AURA_REMOVED,0x0100000002CBB83B,"Valen",0x51 1,0x0100000002CBB83B,"Valen",0x511,49222,"Bone Shield",0x8,BUFF
2/7 09:21:36.952 SPELL_AURA_APPLIED,0x0100000002CBB83B,"Valen",0x51 1,0x0100000002CBB83B,"Valen",0x511,49222,"Bone Shield",0x8,BUFF
2/7 09:21:36.955 SPELL_CAST_SUCCESS,0x0100000002CBB83B,"Valen",0x51 1,0x0000000000000000,nil,0x80000000,49222,"Bone Shield",0x8
2/7 09:21:43.559 SPELL_AURA_REMOVED,0x0100000002CBB83B,"Valen",0x51 1,0x0100000002CBB83B,"Valen",0x511,49222,"Bone Shield",0x8,BUFF
As you can see, the timing between the AURA_APPLIED and AURA_REMOVED are 7.01 and 6.60 seconds, respectively. The second test involved twice as many mobs. I was glyphed for this test, so at cast there were four charges. This would imply:
T+0: Bone Shield up
T+0.XX: First charge used
T+2.XX: Second charge Used
T+4.XX: Third charge used
T+7.00: Fourth charge used
The exact values for XX depend on the mob swing times (I had around 10 mobs on the first pull and 20 on the second), but overall,
it is clear the internal cooldown is not 3s as is generally accepted. I think it is more likely to be 2s as that allows for swing time (when engaging 20 mobs of the same type, their swing timers generally were synchronized, so it wasn't a perfectly steady flow of hits).
The key takeaway is that the uptime of Bone Shield, sadly, can be 33% lower than was previously believed; rather than 9s-12s, it is more like 6-8s, depending on boss swing time etc. Of course, avoidance is still quite nice and one should measure their own uptime via WoL or other tools, but the generally accepted 3s definitely is not correct.