how long does it take to run out of food you ask.... all depends on the bioload they are usuially fed and how much is in the sump.... so.... 8-48 hours give or take.... dumping water from the tank into the sump would help keep them alive longer...
What i did during extremely long power outages was that I dropped fish food into my sump, it made it dirtier and I had to clean the bottoms later but it would be a ready supply of ammonia and nitrite for the bacteria.