As far as how long food stays in your stomach it depends on what that food is. Some foods that contain simple carbohydrates (such as sugar or white bread or a donut) are relatively fast to digest compared to more complex carbohydrates or proteins. Foods contain a variety of different levels of carbohydrates, fats, oils, protein, fiber, etc., all of which have different mechanisms and rates of digestion. If for example you eat some trail mix, the chocolate chips will be digested rapidly, the fruit will take longer, and the nuts could take several hours. The range in digestion times is large: if the stomach is empty, water will leave immediately and go to the intestines, whereas meats such as beef and pork can take upwards of 4 hours.

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The approximate times the food remains in various regions of the digestive tracts:
Esophagus: 5-7 seconds
Stomach: 2-6 hours
Small intestine: 3-5 hours
Large intestine: 3-10 hours.