Transit time (from entering the mouth through leaving the anus) through the adult GI tract generally is 24-48 hours. In a person with decreased GI motility, it could be 72 hours or longer, but this is not really normal. Transit time varies depending on the foods you eat, your metabolic state, etc. so you can't set a fixed time period as to when food is being absorbed, but I think that to say it occurs over 72 hours would be a bit much, since food usually won't stay in your system, and especially not in your small intestine (where nutrient absorption occurs) for that long.
During the first 1 to 4 hours after you swallow food it is subjected to and broken up by churning, hydrochloric acid, and a few digestive enzymes in the stomach, but no absorption occurs. How long it spends here depends on the nature of the food. For example, foods high in fiber will increase GI motility so that they are passed through the GI tract faster, decreasing transit time.
After this, it is now called chyme and is passed on to the small intestine where digestion continues due to the introduction of pancreatic enzymes, and absorption begins. Absorption of nutrients occurs all along the length of the small intestine, with some nutrients being absorbed more at certain segments than at others. For example, iron is absorbed at the duodenum and proximal jejunum (early segments of the small intestine) and so is absorbed earlier than vitamin B12 which is absorbed at the distal ileum (the last segment of the small intestine).
The liquid from the small intestine is then passed on to the large intestine where the water is slowly sucked out from it as it is converted into feces. Though large quantities of water are absorbed at the large intestine (I believe it is about 2 liters a day on average), no nutrients are absorbed here.
So, since it usually only takes 24-48 hours for food to pass through you and the first 4 and last few hours are spent in areas where no absorption can occur I think saying that 72 hours is required for food absorption is a bit of a stretch, but it certainly does take a while.