When you ding or die.
 

When you ding your mana pops back up to 100%, and when you die your mana regen is the same as when you died. So if I died to a train in Unrest with 66% mana, I pop at bind with 66% mana. If I die kiting orcs in Oasis with 20% mana, I pop with 20% mana, etc etc.

I've always seen 0% mana after death. However, I'm from Vallon Zek and PvP is a little different than blue.