Each stack lasts for 8 seconds, you have to stab yourself again by then in order to go up to the next stack. You can tell if PnG is still active based on whether Torr's forearms are dripping blood. Each stack adds one hit of armor to the recovery frames, so first stab gives you one hit, stab yourself again and that time youll have 2 hits of armor, and the third time gives you 3. In terms of damage, each stack adds 20% damage to that last stack. So level zero you do 100% damage like normal, level 1 you do 120%, level two you do 144%, level three you do 172%
The armor comes out once Ferra Stabs Torr and lasts for the rest of the recovery animation until you can block, however if you cancel out of it immediately then you won't use the armor, so you have to delay the cancel if you want to absorb something and then punish it. Stack doesn't get reset except by time.