Between the dogs and the cats, food on floor doesn't really last.
For food that doesn't end up down a critters goozle pipe, as they used to say at the Christian summer camp I attended, "God made dirt, and dirt don't hurt"
My five second rule boils down to what is the visible condition of the floor, what normally happens on that floor and is anything stuck to the food? If the floor looks clean, does not generally have a lot of crazy gross stuff going on and the food doesn't have anything sticking to it, it's all good.