It's very important how you breakup your workouts throughout the week, much of it depends on how much time you have to workout and how often you can get a workout in. If you can only workout a couple of days per week then over training will most likely not be a problem and you will even be able to do your entire body each workout with plenty of recovery time between workouts. If you're like me on the other hand, and get some sort of a workout in most days of the week, then you need more of a strategy so as not to overwork muscles or become run down.
The first thing to remember is to listen to your body, if muscles are soar then leave them alone, they have not recovered yet. Generally a muscle that has been trained to failure will take at least 48 hours to recover in a healthy and somewhat younger person, with older age it can take even longer. If you normally have lots of energy and your body is saying it just wants to take it easy then again it probably needs a break for a day or two. Getting a good sleep and eating healthy foods/supplements is also very important for recovery.
It also depends on the intensity of your workouts. Are you training a body part or muscle group to failure or are you doing a workout that works a muscle more for speed and endurance? Workouts that are short, explosive and focused will strain muscles more and require some rest days between hitting them again, while workouts that are longer and more paced out will tire the muscle but in most cases the muscle will be ready to go again in a much shorter period of time.
For me I plan my workouts day by day because I just never know what to expect since I go to a CrossFit Gym and I find out the workouts for the day when I get there. I have no set routines and I like it this way because my body is always guessing and my workouts never become boring. I take a look at what body parts I did the previous days, what muscles are sore and then I work what ever muscles are ready to go and perhaps have not been trained too much the previous days. I try to do at least one upper body muscle and one lower body muscle each workout but this is not a rule I stick to all the time. Also keep in mind that your genetics play a role in how you need to hit each muscle, you may be OK hitting a genetically strong muscle once a week while other weaker areas need several higher intensity workouts to gain strength and body balance in them.
Make sure to watch my YouTube Video on this subject as I cover other areas related to Workout Routines and regularity:
