Believe it or not, all the building takes place when you are resting a recovering when it comes to working out and building muscle. So many people over emphasize working out and under estimate the importance of rest and recovery.
When you have a body part you want to bring up, it is easy to go into a mindset of an overabundant training frequency. The danger in that of course is overtraining. Some believe it is a myth, but rest assure it is not.
Overtraining occurs when the body is not allowed ample time to rest and recover, therefore any kind of muscle gains and fat loss is hindered (plus you feel like crap).
Overtraining is much easier to accomplish when workouts are longer and/or more intense. The more you put the body through, the more rest it will need. Split workouts usually allow a person to work out more days of the week because different body parts are being trained every day, allowing rest and recovery to take place more frequently. But one should still take note and give themselves a break even if they are doing split workouts.
HIIT (High Intensity Interval Training) and CrossFit are forms of training where overtraining can easily take place. These types of workouts should be spaced out more and have more rest days due to their intensity.
But how long should one wait when it comes to the split workouts, especially if a person does superset training where they are training two muscle groups in the same session?
The answer is in regard to more than just drained sugar levels or lack of ATP in the muscle, but actually due to soreness. Studies show it takes 48 hours in most individuals for muscle soreness to reach its peak. If an individual attempts to train with weakened, sore muscles, the chance of injury goes way up. And you cannot train in the gym if you are injured, at least not that effectively. So, there is not a whole lot of scientific stuff behind the 48-hour rule, other than just do not workout if the muscle is tired or sore because you can get hurt, and getting hurt sucks.
So, as much as you want those calves to get bigger or biceps to pop out in a month, give them a good 48 hours to rest and recover.