For myself I can't quantify the variable but I'll know when it's too lethal when I see it. Foremost is the DM's attitude about it. If he boasts of his PC kill count, he won't be my DM. If he fondly remembers past PC deaths as a personal accomplishment as opposed to congratulating great play of heroic sacrifice, he won't be my DM. If he relishes the prospect of killing a PC, he won't be my DM. If he boasts PCs can only survive if they're very smart and can make no mistakes whatsoever, he won't be my DM. If he declares there's no Raise Dead/Resurrection and tells potential players to suck it up, he won't be my DM. Clarification: it's the "suck it up" that's the deal breaker, not the lack of Raise Dead itself. If a PC dies every or almost every session and it's not of the same player who's just being stupid, he's no longer my DM.