Survival with only water but no food
On average, it has been stated to be about five weeks however it differs from person to person based primarily on physical stats.

The process that would occur :


First food that remains in your stomach is digested
Readily available energy is then absorbed (This includes glucose, carbohydrates, anything that generally doesn't mean fat)
Your body then begins to convert fat into a source of energy. Your body would then begin to lose the resources to make protein and you would experience various effects such as night time blindness due to vitamin deficiencies
As you lose salt and other minerals the water you consume begins to build up in your cells via diffusion
You can either die of not enough energy or lack of protein or too many cells disintegrate from too much water


Source(s) : Biology book, Internet, Knowledge.