The idea of an "anabolic window," or a small period of time post-workout when added protein will translate more directly into muscle gains, has been popular among bodybuilders for decades. However, recent research suggests that it's not as short, or as important, as once thought.
"Even if you delay your post-workout meal for a couple of hours, your body is still using the amino acids from your pre-workout meal to stimulate maximal growth and recovery," explains Krissy Kendall, Ph.D., in the article "Is the Anabolic Window Real?" "It certainly doesn't hurt to throw back a protein shake immediately after your workout, but you can still gain a substantial amount of strength and size even when delaying post-workout nutrition."
However, especially if you tend to train in a fasted state, where you haven't eaten for several hours before training, there's an argument for having that shake as quickly as possible after training.
"If the ritual of having a protein shake right after exercise helps you remember to consume that protein at all, then do it!" recommends Douglas Kalman, Ph.D., in Bodybuilding.com's Foundations of Fitness Nutrition Course. "If you can have both a protein-rich shake and a solid meal in that so-called 'anabolic window,' all the better. By contrast, waiting for several hours before having any protein doesn't offer any benefits."