Tangs, algae or bicolor grazing blennies, damsels and wrasse seem to go a couple of weeks or even months (ie 6 line wrasse, algae blenny, some damsels) without direct feeding as they can forage off the live rocks, ie pods, bristle worms, algae etc. Dottyback, royal gramma, fang blennies, triggers need constant feeding, else they loose body mass. In my situations when I go on 2 week vacations, dottyback/gramma didn't starve to death directly, but seem to have died to fighting as they seem to become more aggressive over territory and food. I'm not too sure since they seemed muscular when they died. In those cases, I miscalculated the volume of food in my feeder, or the batteries died.