Your training week should consist of several key workouts that target a specific training response. In other words, there should be several workouts that are truly “work”. It is not necessary nor productive to smash yourself every day but it is very necessary to smash yourself with regularity provided you can recover adequately.
Creating space between key workouts is the best way to maximize the benefits of those key workouts. Generally speaking, the higher the intensity of a workout, the longer the recovery period should be before the next high intensity workout. Similarly, the lower the intensity of a workout, the less recovery time is required.
At the low end of the spectrum, workouts that apply stress at or below your first Ventilatory threshold- often indicated by a Zone 2 rating or a perceived effort in the 65-70% window, require very little recovery and can be done multiple times daily (for multi-sport athletes) and definitely on back to back days. Recovery times may need to increase if these workouts have extreme lengths. Impact load due to running is another factor that will play into the length of recovery even at lower intensities but generally speaking, you can back up workouts at this intensity with a high degree of frequency.
At the high end of the spectrum, workouts that apply stress at and above your second ventilatory threshold (also referred to as your anaerobic or lactate threshold) can require 48-72 hours to fully recover. One of the biggest mistakes athletes make is not leaving enough space between high intensity workouts. Injury and overtraining are two of the most obvious consequences to this habit but there is a third consequence that is perhaps less obvious but equally important. When you don’t create enough space, over time it becomes very difficult to put out the effort required to actually trigger the training response you are looking for. When you do leave enough space you put yourself in a position to physically and mentally apply an appropriate stress load and thus trigger the adaptive response you are looking for. This means you may only need to do 1-2 high stress workouts in the week. Highly trained individuals or professional athletes who have less additional life stress may be able to do one or two more high intensity workouts simply because they have more time to recover.
Creating space also applies to your daily routine. If you are juggling two or more training sessions in one day, consider spacing them out as much as possible. You are better off doing one of the workouts first thing in the morning and the second deeper into the day than to lump them both together into one epic session. Creating space allows for physical and mental recovery, which will ultimately allow you to execute with more intent during each individual session.