On average, its around 4-5 hp but its mostly an equal range of 4-7HP. But that's why I discussed the 18 damage attack, because if they recovered 4HP from 0, they're left with an "insta-kill buffer" of 17. 18 damage is more than enough to instantly kill them. Of course, they'd be down from 4 damage, but I'm talking about them being wholly dead, not merely unconscious or unstable.
And that's a fighter which has an unusually high maximum HP on average compared to other classes. A wizard or sorcerer might only have 10HP to buffer an instant kill.