Depending on the type and quantity of it, your food usually takes 24 to 33 hours to complete its journey in your body. So when you are having today's lunch, your digestive system is still working on the lunch that you ate the day before.
Food spends about 3 or 4 hours in the stomach it then takes it about 3 more hours , sometimes more to move through the small intestine, After that it moves into the large intestine where it can stay for up to 36 hours dependant the food you ate. so unless you swallow a laxative of some sort, it can take about 40 hours or more.