"How much rest is best? The 'best' amount of time to rest between sets totally depends on the goal that you’re training for! Do you want to get stronger? Build muscle? Or increase stamina and endurance? If you want to get stronger, you should program our training to increase our strength as quickly as possible. For hypertrophy and muscular endurance, the programming that you choose should reflect the type of adaptations you’re seeking to achieve.
To get stronger, you should rest at least 3 – 5 minutes between sets. The reason you need to rest this long is because much of the energy your body consumes during traditional strength training (think sets of 1 – 6 reps) is produced via the ATP-CP (Adenosine Triphosphate – Phosphocreatine) system. The ATP – CP system uses phosphagens to produce energy (ATP) very quickly and without the use of oxygen. Unfortunately, your body has a very small phosphagen reserve, and depletes quickly, in the neighborhood of ~15 seconds. Resting at least 3 minutes between sets gives this energy system enough time to replenish phosphagen stores. You’ll cool down if you rest longer than 5 minutes, so there’s no need to increase your rest periods longer than that.
If your goal is to increase muscular size, then aim for rest periods of at least 1 – 2 minutes between sets. Your typical hypertrophy/bodybuilding training (think moderately heavy sets in the 6-12 rep range) draws energy from the ATP-CP system, as well as the glycolytic system (this is the energy system that is primarily fueled by carbs). The glycolytic system produces a steady stream of energy to your working muscles via the oxidation of glucose (sugar), and can supply your body with ATP for anywhere between 15 seconds to 2 minutes. Resting 1 – 2 minutes between sets also increases muscular fatigue via lactate production, and has been shown to increase short-term strength gains and increases in hypertrophy.
Finally, to increase muscular endurance and stamina, you’ll want to rest anywhere between 45 - 90 seconds between sets. Endurance training (traditionally sets of 15 – 20 reps) draws much of its energy from aerobic metabolism. This means that your body burns both carbs and fat in the presence of oxygen to produce ATP. Typically, the goal of endurance training is to increase muscular stamina and fatigue resistance, and regularly lifting weights in the 15 – 20 rep range improves your body’s ability to buffer and clear lactic acid, as well as increases capillary density in the working muscles. "