How many days does it take for a cat to change its food?

Cat diarrhoea can be self-regulated in 2-3 days. Cat diarrhoea may be caused by the cat not adapting to the new cat food. Stop feeding the new cat food, fast for half a day, feed more water and feed some probiotics to regulate the gut. Change to new cat food gradually, mixing the new food with the old food and adding less and more to the new food.