As noted above, it depends on your stamina levels.
Here's a few suggestions on absorbing boss mob damage;
Revenant;
Start combat with full stamina, block two first strikes and let him hit you with the third. While you are attacking him, you will be able to get back to full stamina to redo the cycle.
Arcane Horror;
Stay away from it (I see so many people with aggro run up to it, which simply means it starts teleporting), so that it will only toss one projectile against you and not teleport around. Let the DPS do the job while you shieldwall block the single projectiles.
When he does the 3x3 barrage move, feel free to move back and forth to avoid them. You can shieldblock about 60% of the barrage if you have full stamina, which will grant you a full guard bar.
Red Templar Commander;
His normal swings costs you same level of stamina as blocking the Revenant. I will typically melee him and absorb two normal strikes (Which gives full guard bar) and then start the kiting while the DPS do their job. When he is not spinning, doing the ground smash, or jumping at you, you can trigger his normal strikes still to regain more guard in case you lose it do to stray Archer arrows.
Daemon Commander;
Keep your stamina near max at all times. In order to keep a high level of stamina, alternate between blocking his normal slash to gain Guard, and letting him hit you to spend it. This will leave you with enough stamina to shieldblock the damage part of his scream, which will give you Guard, and let you have the stamina needed to use Payback Strike to get out of the fear component right away. This leaves you with more time to move into a better position for when he does his 4 bolt ranged attack. (Which you can chose to block if you have the stamina for it).
When he teleports back to you, it is possible to shieldblock his "popup" attack if you position correctly. Once he is back, redo the cycle as he will start slashing normally again.
Venatori Commander;
Not alot to absorb, but you can shieldblock her fire mines, and her Arcane Barrage in order to build guard for when she firewall's.