Usually 14 days from the day the person stops eating or drinking. I have seen them live a litte past this if the person is overweight. Also, in hospice care,
we believe hearing is the last to go. So, please talk to your mother and tell her that you love her, that it is ok for her to go, and that you will be alright. Sometimes, the person is holding on for a family member to come by or call. I have questioned in my career many times, "How long will this go on?" I have seen on numerous occasions, that if the family members or close friends talk to them (via in person or telephone), it somehow releases them to pass on in peace. Otherwise, the person may linger until that occurs.
I hope this helps. I am available for any questions you may have.