Protein is a macronutrient that is essential to building muscle mass. It is commonly found in animal products, though is also present in other sources, such as nuts and legumes. The Institute of Medicine recommends that 10 to 35 percent of daily calories come from protein. How that equates to grams of protein depends on the caloric needs of the individual. According to the U.S. Department of Agriculture, the amount of protein foods a person should eat depends on age, sex, and level of physical activity. Most Americans eat enough food from this group, but need to make leaner and more varied selections of these foods. [1]https://www.livescience.com/53044-protein.html
Most of the protein shakes contain whey protein. Whey protein is a mixture of proteins isolated from whey, the liquid material created as a by-product of cheese production.[2]https://en.wikipedia.org/wiki/Whey_protein Whey is a “fast-acting” protein; its absorption rate has been estimated at ~ 10 g per hour.[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5828430/. A typical protein shake contains about 15 g dose of whey, which means that with this rate it would take just 1,5 hour to fully absorb a protein shake.