The honest answer is a range, not a clock. Dying from illness or old age usually moves through a long decline measured in months to weeks, a transition measured in days, and a final active phase measured in hours to a few days. No one can give you the exact hour, and anyone who does is guessing.
This page gives the ranges anyway, because "we can't say" is not good enough when you are sitting at a bedside trying to plan. Below are the timeframes hospice families ask about most: the whole arc, the active dying phase, and the specific questions, how long after eating stops, how long after the death rattle, how long the breathing changes can last. For what these stages look like at the bedside, read what dying actually looks like.