The rate at which heroes die depends on a couple of things. First of all there’s the outside influence, whoever you’re fighting and how strong they are. Since this would influence all your heroes equally I’ll leave that part out
Then it’s a combination of the shield and the heart icon. The higher the shield/defense points are, the less damage they receive from an original hit. Whilst the exact mechanics and percentages aren’t fully known (or at least by me) - the higher this number is, the better they can withstand damage (oversimplified: if someone hits with 10 power, and your defense would be 6, you’d only be hit for 4).
Heart/hitpoints is the other part that influences this, as the more you have of this - the more hits you can take before you die. It is therefore a combination of the two
The speed at which your heroes charge mana doesn’t influence how quickly they die (unless they’re a healer, cause then they could heal themselves faster). The speed at which opponents charge can influence, as they could hit you more often and kill you faster.
Hope this helps!