This must be a very intense time for you. The length of time someone can live without eating depends on their weight (or amount of body fat), health, and, many Hospice care can be extremely helpful at times like this, medically, but also emotionally. If your mother isn't consciously ready to die, but has stopped eating for some other reason, such as painful swallowing or depression, please talk to her doctors right away to get help. My thoughts are with you.
believe, attitude. Medical experts usually say the average person can live for 4 to 6 weeks without food. The amount of time someone can go without water is much shorter, roughly a week (or less). If your mother is ready to die, she may go sooner. You don't say much about your situation, but I hope you and your mother have the support you need.