How much time will it take for negative consequences to occur from a layoff from training? Generally, in as little as two weeks duration of a layoff will result in some strength and muscle loss as well as decreased aerobic capacity. The old adage of "use it or lose it" is in play here. The exact amount of time it would take for noticeable strength and muscle loss to occur is dependent on many factors (i.e., age, workout experience, metabolism, workout frequency, your health, etc.). It all comes down to muscle memory. The more advanced your are at working out, the more muscle memory you will accrue. This means decrements in strength and muscle mass will not be as significant relative to a neophyte. The more experienced you are at working out (i.e., more than a year) and the more frequently you train (i.e., at least three days per week), the less the loss of muscle strength and size as well as cardiovascular capacity. Of course, no matter how experienced you are, your age is a big factor that impacts how fast you lose your gains. The older you are, the faster the loss of muscle mass and strength. Remember, your body doesn't need muscle mass to survive, especially as you get older. If you're taking time off because you're sick, you'll likely lose more strength and muscle mass than when you're healthy.
So how can you lessen the loss of strength that will occur when taking a vacation? No surprise here. You need to perform at least some light resistance training and cardio. This means lifting light weights and going for some brisk walks. If you injured an arm or leg, you can always work around the injury. Swimming is an ideal exercise in the event an injury has occurred. Most hotels have a gym nowadays. The important lesson here is to keep moving to reduce joint stiffness and immobility.
BOTTOM LINE: Some form of exercise is better than no exercise at all!
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."