There is no internally consistent answer to that in the Dresdenverse. Other than to say that your dead when JB says you are. JB is selling the idea that what makes you, you, is separate from the body. So are we talking about the death of the body or the death of the soul?
For Harry, obviously his body didn't decompose, which is an indicator that it didn't die. And in point of fact it was healed. That is, he no longer has a chest wound, when he awakes in the well. And if you believe the timeline he didn't leave his body until it was healed. How much time elapsed before he woke in Chicago over Chicago?
I'll grant you that is a long time to heal, but JB needed that time. It might have been more fun if JB had played with an alternate timeline where Harry's death was final. Kinda like Scrooges ghost.