I typically feed mine about once a night since my rats like to move around at night. If it's still there in the morning I typically throw it away. I just don't know if the rats have peed/pooed on it. I know people say rats eat their own poop and that they'll stockpile food in their litterboxes, but I don't want to encourage a rat eating their own poop... I personally find that unsanitary, the rat is obviously unhappy about something, and it's just a great way to start an illness in the rats.
Anyways, I digress. I may give them just a little bit less the following night if food is left over.
My rats generally like to take all of their food bowl and then... stockpile it somewhere else. Can't tell you how many times I've lifted a hide up just to have a horde of food in front of me.