How long of a rest period should I take to increase muscle mass?

You should aim to rest from 0 to 60 seconds between sets. Rest periods will depend on how much weight you are lifting. Your body has three energy systems-Adenosine TriPhosphate-Creatine Phosphate (ATP-CP), glycolysis, and oxidative. The oxidative phase requires oxygen and is used primarily for lower intensity longer duration activities such as walking for 20 minutes. Glycolysis does not require oxygen and uses the breakdown of carbohydrates to produce energy. This energy system is used primarily in moderate to high intensity exercises such a set of eight to twelve repetitions. ATP-CP is used for high intensity, short duration activities usually lasting for less than 10 seconds. The goal and the intensity of the exercise will determine the amount of rest you will need to take. In order to maximize muscle growth, you will need to work out with 75-85% of your one repetition maximum for six to twelve repetitions. Based off of these two factors, you will be using primarily your glycolysis system for energy production. You can adjust your rest period based on this. In order to completely recover from maximal effort, you would need to wait up to five minutes. But, you will be training between 75-85% of maximum effort, so you do not need 5 minutes. A rest period of 0 to 60 seconds will be best to stimulate muscle growth.