Deb, Sorry to hear about your mother's condition. Let me first address the time left question. The 6 month is just an estimate of an average and is based on data that is at least 5 years old. That means it could be shorter or longer, in some cases much longer. If she/you choose not to have further treatment then hospice or palitive care will be used to keep her comfortable and minimize pain. You can learn more about this by consulting the social worker at your mother's nursing home who can put you in touch with the people that provide the care and they can answer most of your questions.
If your mother has dimentia it will be hard for her to understand the treatments and make them more difficult to adminester. My 94 year old MIL has dimentia and used tio ask if I was better everytime I came home from chemo despite the fact that I flet terriable. She was unable to understand the concept of cancer and that chemo made you feel worse to get better in the long run. I don't think she would have been able to sit through more than 2 hours of chemo let alone a 10 hour session. I had to take her to chemo a few times and it was always a chore to watch her and get chemo.
I know that his is a difficult and heart wrenching decsion that you and your mother have to make. Take time to talk to all of thise involved and when you feel comfortable with a desision make it and do not look back. Good Luck and you are in our thoughts and prayers.