The right time to die...

How to decide whether one of your characters shall die?

I have mixed feelings, she's a haunting presence, bound to a med-unit, kept artificially alive.

Lots of emotions are growing up among other characters too. There is tension, and tension is good, people say.

And the situation will haunt other characters for quite some time if she's left hanging in there, fighting for her life.

So...when it is the right time to die?