Your Bleed Timer is how long you have to live after you are reduced to 0 Hit Points and start Bleeding Out. When your Bleed Timer expires, you die, and travel back to the borders of Veilguard to Respawn.
Your base Bleed Timer is 2 minutes, or 120 seconds.
While you are suffering from the Envenomed condition, your Bleed Timer is reduced to 1/6th its normal duration (or 20 seconds for a base Bleed Timer).
The Vigor ability increases your Bleed Timer by a set amount for every level of it you take. Other effects such as the Clotting Elixir or some enchanted items can also increase it. All of these effects stack with each other.
If another character begins performing a healing spell or ability on you, your Bleed Timer is paused while they are doing so. If they are interrupted or for some reason are unable to heal you, your Bleed Timer resumes its progress.