http://imgur.com/a/n7T0y
Just a small snippet of three different events to talk about. Shield Block up time is affected by haste, but is also highly affected by play style. You need to ensure every shield block gets extended at least once while maximizing your chances at multiple extensions.
First picture, you shield block without shield slam up, devastate, then need to leap away for rot. Need to watch timers, don't shield block shortly before leaping away. Also you missed a GCD in here with devastate going off at 26.312 and leap going off at 29.757. Very important to never miss a GCD for maximizing chances at resetting shield slam and casting shield slam.
Second picture, you cast shield slam and then shield block. This is a common trap. You have no rage, you shield slam, you have rage, you decide to shield block. No No. You need to sit without any defenses up until shield slam comes around again. Then you shield block and immediately extend it with shield slam. You got lucky here and proc'd a shield slam on second global after shield block, but if you didn't get a reset then this shield block would have not been extended because CD on shield slam is more than the duration of shield block (until about 24% haste, which is why it's often touted as a break point... because it allows for lazy play where you can always get at least one extension into a shield block no matter when you cast it. Though if you continue to play well then you are guaranteed 2 extensions for each shield block).
Don't cast thunder clap ever on single target. It doesn't reset shield slam, it isn't a shield slam, don't use it.