When leaving the house, always leave your cat with a decent supply of water.
Even short-term dehydration can be fatal for a cat.
The average adult cat can survive without water for about 5 days.
But sometimes even two days of dehydration can be fatal for a cat.
How long the cat will last depends on the room temperature, the health and age of the animal.
A small kitten may not survive even a day without water.
An elderly cat with weakened health can die within a day or two without water.