There are so many different opinions on working out and how much is too much. There are those who say you need lots of rest, and minimal workouts, while others tout that you should rarely take a rest day.
The way our muscles grow is we stretch the muscle fibers and put them under enough tension so that our muscle fibers get tiny tears in them. Then during the resting phase, our muscles repair themselves and build new muscle fibers over the tear (kind of like a scab). This makes the muscle denser and stronger.
Now our bodies need about 72 hours for the torn and stretched muscles to heal.
Typically if you are doing a workout routine, you workout a different muscle group on each day of the week. Using this method though we typically hit each muscle group only once a week.
That is not enough for really great gains. You need to be working out muscle groups at least twice a week. To do so, double up your muscle groups. For instance instead of taking a shoulder day and then a leg day, combine the two. By doing this you can get all your muscle groups done in three days. For example: Day 1 — Chest and Triceps. Day 2 — Back and Biceps. Day 3 — Legs and Shoulders.
Then you can repeat those days. You are still getting 72 hours in between the muscle groups being worked out, and yet are getting in two workouts on that muscle group each week.