There are a number of variables that can, and should, change from workout to workout. The most important of these variables is intensity. Not every workout has to be brutal. In fact, high-intensity exercise should ideally be limited to no more than two or three times each week. This becomes more important the older you get. It is often far more beneficial to do a light workout. Any day following an intense workout should be lighter (if it isn't a day off). Think stretching, rolling, and light cardio. Yoga and pilates are great! Rebound from sickness or injury by testing the waters with a lighter workout. But light does not mean non-existent! The hardest thing about light workouts is keeping them mildly challenging. You can take longer rests between your normal exercises, or do added repetitions at a moderate weight. Work on form or technique. Keep the resistance lighter and add an element of balance (It is called proprioception in trainer-speak). The possibilities are endless.
Workouts can also be broken up into different activities or muscle groups. These are the most common varieties of split-routines, whereby an individual maintains a preferred intensity but cycles through different foci. The concept here is to train the whole body at least once a week but provide adequate recovery time before hitting the same area again. A common mistake of many who follow a split routine is to focus on only one component of their exercise program. A complete workout has components of, and should challenge; strength, speed, balance, endurance, flexibility, and neuromuscular control. With this type of routine, it is important to maintain intensity levels throughout the entire cycle of workouts, or the body may develop imbalanced movement patterns. Some of the more common split routines are: upper body/lower body/rest, strength day/conditioning day/rest, chest and shoulders/legs/back and arms/rest, sport day/workout day/weekend rest, crossfit day/yoga day/rest, boxing day/jujitsu day/weekend rest, etc.
So now you have a general idea of how many days a week to work out (and take off), and a few ideas of how to use different workouts to complement each other. We can now turn to the final component of a long-term fitness program; planned recovery breaks. This often-overlooked component in the fitness world is included in every competitive sport. Think about it as an off-season or a vacation. The physiological purpose of this vacation is to let the body have an extended rebuilding period. During this time, stores of vital minerals and nutrients that are gradually depleted by intense exercise can be replenished. Tendon and ligament breakdown can be reversed. Mental and emotional energy will be restored. Simply put; let your body refresh itself.
Plan on a week or two off for every six months of continuous exercise. I often tell clients to align it with either a vacation or the busier season(s) of their professional or personal lives. Taking time off can be very difficult for some people. If that is you, it is best to frame it as allowing your body to excel at the subconscious work that it is trying to do all of the time. And unless you are on a dream vacation and spoiling yourself to the fullest; this prolonged rest period is not a reason to deviate from adequate and healthy nutrition. When you return to your exercise regiment, build slowly back into the intensity and frequency of workouts that is optimal for your goals and conditioning.