Unless you have a huge free feeder and only one cat, there is no way for the food to go bad. It would have to sit out for days to weeks for anything wrong to be happening with it....for goodness sakes, cats eat dead rodents. lol
Our cats will also turn up their noses at times if there is food left and it is time to feed again. Just because the food loses its smell, doesn't mean it cannot be eaten or is bad.
This is one reason why it is good to keep the bags closed, so the smell lasts longer.
We put down food in the morning and at night to keep it smelling fresher, but if some is left over and they are screaming in the morning for more, when there is some left, we just mix fresh in with the rest and everyone is happy.