When you start exercising, your body undergoes several adjustments. Depending on the type of exercise, this person should experience an increase in strength, a decrease in fat mass, an increase in muscle mass, an increase in insulin sensitivity, a reduction in blood pressure, and an increase in cardiovascular conditioning (VO2 max). All of these physical enhancements are referred to as "workout adjustments." Training adjustments will last as long as you train hard enough to sustain them. If you take too much time, they disappear.
You can see a reduction in muscle size in just one week! Generally you mostly lose water. Your strength gains will not go away for some time, as strength is primarily a neurological phenomenon and muscle size is not necessarily proportional to strength (just look at the size of some Olympic weightlifters). According to a recent research report, you notice an increase in blood pressure and a decrease in cardiovascular fitness within 1
Fortunately, there are ways to reduce muscle loss when you're not exercising! When dealing with an injury, you can focus on training other parts of the body. Even if you can only train your arms, you do so because you still see benefits. When your time is short, three to five 10-minute workouts per week are enough to get a good level of fitness and stamina. In this case, the key is to hold the intense workouts with minimal rest. When training is out of the question, focus on your diet. The more protein you consume, the less your body will use the protein in your muscles for repair and energy, reducing the amount and speed of deterioration that occurs when you are deconditioned. So if in doubt, eat more protein!
Moral of the story: Use it or lose it!