I have been looking around and I see some claims of 15 to 30 minutes. Googling 'snake bite fastest time to death' brings back many different answers. In one place I see Black Mamba with 30 minutes and in another it takes hours.
For the sake of this question, assume time to death is when antidote will no longer help. Person might still be breathing, but are those who are unlikely to live, regardless of what care they receive(Triage)