Are you talking about when you put it down for them to eat or are you talking about leftovers in a can? Put down for cats to eat can be 12 hours or so before replacing. Leftovers should never be left in the original can as once opened the metals inside the can may react with the food. You can refrigerate in a plastic container and, when ready to serve, put in microwave for 10 to 15 seconds to bring it to room temp for the picky kitties.
Assumptions do nothing but make an ass out of u and me.
We can stick our heads in the sand for only so long before it starts choking us. Face it folks. The pet population is bad ALL OVER THE WORLD!