The whole discussion shifts if you can reach 'unhittable'. Once you get the magic 100% (or 102.4?) combined b/p/d/m then you can factor in block as a given reduction on physical hits when calculating how much stamina is 'enough'. Until you get to that point however, 'enough' stamina is going to be loosely defined as whatever the biggest unblocked spike is.
I don't think it's helpful to focus too heavily on Shannox, but I also don't think you can say that you can mitigate all his spike damage through cooldown usage. Sure, you know he'll do a spike right after he gets his spear back, but that isn't the only time he's going to do a melee and arcing slash combo and it's the ones in between where you also have the DOT to factor in.