A body can live without food until the fat stores of the body are depleted, hence for an underweight individual, that might not be very long. For an overweight individual, it could be weeks going into months even.
A body cannot live with water however, and I seem to recall a figure of about 5 to 7 days before death would occur.
God I have mention Army survival training twice today!!
I recall "the rule of 7"
You will only survive......................
7 seconds without blood (i.e arterial vein)
7 minutes without air (before guaranteed brain damage)
7 days without water
7 weeks without food
7 months without a sh4g
I think the last one was a joke, but believe the rest are a "rough" guidline