This is only if you are under the effect of multiple critical hit enhancing buffs (e.g., Battle Litany, Chain Strategem, Spear). Only if you've snapshotted 2 or more of these buffs onto your DoTs do you use PP at 2-stacks. Otherwise, it's 3 stacks (and especially if you have low Crit to begin with - low Crit = low chances to proc).
A 2-stack PP is 240 potency. 3-stack is 420 potency. Unless you've enhanced your Crit rate enough that your DoTs have a high chance of simultaneously procing, you lose potency by using it at 2 stacks. With no Crit buffs aside from SS snapshotted onto your DoTs, you can't bank on using it at 2 stacks because it may double proc and you may lose a stack.
2 2-stack PP = 480
1 3-stack PP = 420
It's a loss of 60 potency if you overcap, but if you do 2-stack + 1-stack because you never got that second proc -
1 2-stack PP = 240
1 1-stack PP = 100
It's a loss of 80 potency from not waiting until that 2-stack was a 3-stack.
General PP usage—
No Crit buff on DoTs: 3 stacks
Battle Litany OR Chain Strategem on DoTs: 3 stacks
Battle Litany AND Chain Strategem on DoTs: 2 stacks
Battle Litany AND Chain Strategem AND Spear (AOE or ST) on DoTs: 2 stacks
I know there was discussion about Battle Litany/Chain AND Spear, but I think the BRDs still said 3 stacks for that one since +5% crit for an AOE is kind of eh (it’s not as great as Litany or Chain’s boost). However, I think if the Spear was a single-target card (10% or 15% with an enhanced) on you, AND if you have Battle Litany, Chain, OR BOTH, on your DoTs, you use PP at 2 stacks.
This is all min-maxing territory. I tried to stay away from too much min-maxing because that's not what the OP is currently after. Generally, it's used at 3-stacks; 2-stacks if you're in an AOE situation with multiple mobs DoT'd OR if you have snapshotted multiple critical hit enhancing buffs onto your DoTs.