Every 3 - 4 days is fine. I'm away for 5 days now and expect all the tank inhabitants to be hungry but fine when I return tomorrow night. For longer trips I tape baggies with the proper amount of food to the tank and instruct my tank sitter to feed ONLY ONE BAG per visit. It's the only time I rely strictly on freeze dried food, but I'm going to ask someone to thaw some frozen food and feed that.
Do you have an auto top-off system for evaporation?
If you have a skimmer you also want to teach your tank sitter how to empty the cup if you don't have a high capacity drain.