Shackling and Kara mobs

[Grendeel]

I know the length of the shackle is very random from past experiences.   I think we have developed the strat of shackling, then casting twice, then reapplying shackle. 
I found this researching priests.  Not sure it helps or not but here it is

Q: My guild is having me Shackle Undead in Karazhan. What Spellhit/Spellpen do I need?

A: Shackle Undead is a Holy spell. Since there is not a Holy Resist stat, Spell Penetration gear will not help you. The level 71 Undead require 4% Spellhit on your gear to minimize breaks. This is equal to 50.4 Spellhit Rating. Moroes' level 70 adds only require 3% Spellhit (37.8 rating) for maximum effectiveness. For more information on how resists work in general, see the Stats section in this guide.

Not everyone agrees with me here, but I don't recommend casting Shackle "every few seconds." Each periodic check is the same as the initial resistance check. Instead, start casting and cancel repeatedly if you want to improve average response time to a break. Better yet have a Mage or Hunter stand ready to freeze the mob as you reapply Shackle. Also, lower Rank Shackle is no more likely to break than top Rank...but with proper Spellhit your Shackle will often outlast Rank 1's 30 second limit.