Many of us are guilty of over-feeding our fish. We get used to watching food disappear once or twice a day within seconds of feeding, and feel the guilt trip every time you walk past your tank and see your fish following you from corner to corner. The truth of the matter is that our fish are generally pretty well fed. The consistent presence of nitrate and phosphate in a system is an indication that our fish do not go starving.
Typically speaking, saltwater fish can go without food for at least 4 days, but can, if required, go for up to two weeks without being fed if you believe what you read in the forums. Of course, this is determined by the type and the size of the fish, as well as their established feeding regime. As an example, Anthias are regularly mentioned as fish that require multiple feeding every day, while tangs are happy to be fed one or twice a day. I keep Anthia, and like most people I go off to work on a daily basis so cannot be there to feed them constantly. They have become accustomed to feeding twice a day, and as the fastest fish my my tank they have also become some of the fattest!
Knowing that my fish are well fed, I am comfortable with arranging for family to drop in every four days to feed. I know that the lack of feeding will not be detrimental. It is worth evaluating the condition and feeding habits of your fish to determine how many days you would be comfortable to leave them before you feed saltwater fish while on vacation.
In extreme circumstances, I believe that larger fish can go for weeks without food. It is with experience that I make this statement. My Hippo Tang got swim bladder disease, and until that point was a healthy, fat example of his kind. While I had it in QT I tried many medications hoping that something would help to correct the disease. Unfortunately, after 4 weeks he became lethargic, due to lack of food. I had no choice but to euthanise him. I had learnt an important lesson, though, that fish are far more robust than we give them credit for.