Four days would be about the maximum I would leave my cats alone. They have five litter boxes for the three of them, and I would also leave out lots of food and extra water. Any longer than that, though, and I have my grandparents come in and fill their dishes, clean the boxes and give them some play time

But, I don't have a problem leaving them alone for a weekend. They have each other for company, and have never done anything to cause me to worry.