A few weeks
There is a simple rule for this, that stems from survival situations. It is a general rule, but it can depend on several different variables. It is the rule of few. You can survive a few minutes without oxygen, a few hours with too much/little heat, a few days without water, and a few weeks without food. In general, though, it depends on variables such as previous nutrition, amount of excess body fat, how much work will need to be done over this time period, etc.