Your body is a miraculous organism which has the capability to adapt to physiological changes that are put on it. This means that if you subject your body to an intense bout of resistance and cardiovascular training, your body will essentially "remember" the effects of this stimulus after a break in training. These effects include the pumping of blood (and oxygen) throughout your body to affect your metabolism during exercise. Training-induced effects include ameliorating your blood pressure, blood cholesterol and blood sugar levels as well as increasing your muscle fiber size and neuromuscular adaptations. All of these favorable physiological effects will dissipate to varying extents as a result of sedentary behavior. The longer the layoff the longer it will take your body to regain the loss in muscle mass and cardiorespiratory capacity. The good news is that your muscles (including your heart) have a "memory" of how it felt as a result of resistance training and cardiovascular exercise. Thus, no matter how long your layoff period, you can certainly regain some, if not all, of your muscle mass and cardiovascular (aerobic) capacity. The more fit you were prior to your layoff, the more gradual the decrement in muscle mass and aerobic capacity.
To lessen the effects of detraining when going on a vacation or when you're more busy at work, try to get in at least some exercise. In other words, it's better to perform some exercise (i.e., workout at least once per week) than to not exercise at all for a period of time. When time is short, increase the intensity of your workouts by performing abbreviated weight training (i.e., 30 minutes) and high-intensity interval training (i.e., 1-minute sprints followed by 1-minute slow jogs for 10 minutes). If you injured one limb, you can still exercise the rest of your body to minimize the detraining effect.
BOTTOM LINE: In most cases, there is no reason to not continue exercising no matter the changes that may occur in your lifestyle or circumstance. The important thing is to continue working out and not stop altogether in order to stem the detraining effect on your body.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."