You can check your liters via the Leaderboards.
It will come relatively quickly if you are proactively sucking blood. You will learn to suck blood during Chapter 1.
I just made it a habit to suck blood anytime I was hit, its relatively easy to do during battle even in the midst of combat and you can blood suck an enemy multiple times until it dies, you just have to wait for the skill to cool down. Sucking the blood from the impaled soldiers also count towards this achievement.
Also you DO NOT have to be hurt to be able to suck blood.