How long after eating food is it converted into calories?

Food is broken down in a process called respiration. Enzymes are usually involved in the breakdown of food. Proteins are broken to amino-acids by protease, Carbohydrates broken down to simple sugars e.g fructose and glucose and Lipids (fats & oils) are broken down to fatty acids and glycerol by Lipase. Once they are broken down, they are taken to the cells where they're respired to produce Energy, Metabolic water and carbon-dioxide.