WAR - 80~95%, similar to a MNK, almost always right when damage is dealt.
MNK - 80~95%, always when the first damage is dealt (usually waits until 90-95% if the first damage doesn't do enough).
WHM - 0~20%, if taking too long to kill; killing fast will cause it to not Benediction, even without stuns.
RDM - Never really observed, useless 2hr.
BLM - Never really observed, useless 2hr.
THF - 20~50%, usually tries to 2hr right around 50%.
PLD - 20~50%, usually tries to 2hr right around 50%.
DRK - Never really observed, useless 2hr.
BRD - 80~95%, but lolSV.
RNG - 20~50%, almost impossible to avoid unless you instantly kill it during a single stun.
BST - Never really observed, but about halfway?
SAM - 50~80%, I think.
NIN - 0~20%, if taking too long to kill.
DRG - 80-95%, similar to a MNK, almost always right when damage is dealt.
SMN - 0~50%, they are weak so it's very easy to prevent 2hr at all by WSing @ 50%. I noticed they seem to 2hr about 2-3 seconds after reaching 50%