Suicide by sitting in running car in garage...?

I have a close friend who is severly depressed & I fear she might try to commit suicide. She always talked about the "painless" (any way will cause pain to those left behind) way of going by sitting in a running car in her garage. She has been to counselors & put in a psychiatric hospital before but I don't think it helped. My question is this... how long would it take for a person to die by doing this? I usually go over her house to check on her before work & after work & later in the evening but I fear it is not often enough. I'm scared one day when I show up she will be dead. Does this kind of death happen in minutes or hours or what? I'm trying to be a good friend by going over there as much as possible to keep her from doing anything but I'm scared that if this death can occur in minutes or within two or three hours she could do it while I was at work or at night. She has a small-ish one car garage & a Ford Explorer. Any estimates on how long it would take if she were to do this?