Time For Recovery
Since time spent in the gym is where your muscles get the stimuli that lead to micro tears, it's the time away from the gym where your muscles will be able to repair and recover. A big part of this revolves around getting enough sleep each night, and by enough, I mean at least 7 hours every night.
The time needed for a muscle to recover from the previous workout, repair, and be ready to be worked again at a slightly higher capacity can be up to 3 days (72 hours). If you workout your back on Monday morning, then it would be at least until Thursday morning before you could train your back again. Given that you should be focusing on no more than 1-2 muscle groups per workout, and allowing at least 1-2 days complete rest a week, you should only be training each large muscle group once a week. This means its important to have a training routine that allows each muscle trained adequate time to recover before being trained again, including being worked indirectly along with another muscle group.
Below is a good starting point for a weekly muscle-building schedule:
You’ll notice that I’ve included legs twice within the week, and this is simply due to the impact on muscle growth you’ll get from working them more than once a week. Your legs can move a lot of weight, creating a lot of resistance for the muscles. Exercises like the barbell squat, leg press, hamstring curls, hack squat, walking lunges, stiff-legged deadlifts, and sumo squats, will have an effect on the body’s natural production of testosterone, which is a potent muscle building hormone.
As a rule of thumb, if your muscle still feels sore and you do not think you’ll be able to work them at least as hard as the last time you trained them, give them an extra day of rest. Building muscle is a marathon, not a sprint, so there’s no rush to train them as often.