How to teach your parent with dementia to get out in case of a fire?

Exactly how long is "the few hours a day she is alone"? I leave my own mother alone while I go out to walk and run, but it is during her morning nap time and I seldom am gone beyond an hour. Once they reach a stage where they need coaching on how to escape a fire they are in the same category as a very small child, you can not expect them to remember what to do in a crisis. You need to understand the real dangers of leaving her alone and be at peace with the level of risk you are willing to accept.

You are not going to be able to "teach" her any better than what she knows now. Ask someone to help you be objective about her being alone at home for even a few hours a day - it can be hard emotionally and practically to realize and make provisions for when that is not the case. My mom would not have been able to stay in her home as long as she did without a really, really good neighbor - and she eventually had a fall that ended her independence, but before that seemed way too functional to have known or decided she was not safe, never mind my lack of understanding of what was slowly happening to her at the time.

It would concern me too. Except for the early stage, it's quite risky to leave someone with dementia alone. If she can't demonstrate that she understands that fire is a danger and how to get out safely, then I would believe that she is not able to do that. It's my understanding that you can't teach new things to someone with dementia, since they do not have the capacity to learn new things. I would find someone to come and stay with her while you're gone or take her to a senior center to stay while you're out.