How long can my mother survive in hospice with out eating or drinking?

My dear Mother (who i cared for until last year and is now in a NH) is not eating or drinking much after 8 years of COPD and dementia, I am so sad and can't stop crying as I watch her wither away, how long before she can't live without eating or drinking? Hospice is now involved and I fear she will be gone this week?? Grieving daughter