How long can I leave my cats for on their own?

How long can I keep cats alone for? I am going away for two days this w/end but am worried about leaving the cats alone. They will have plenty of food and water but I don't have a catflap for them to go outside though they can use their litter trays. If they are going to suffer in any way I will make other arrangments but someone told me they should be fine. I'm a bit dubious.