A 2005 study in the Journal of Applied Physiology found that for highly trained athletes, 2 weeks of detraining seems to be the amount of time that muscle atrophy tends to occur. This seems to be because of the differences in the initial muscle fiber size and the conditioning level of the athlete.
“Highly trained athletes have shown preferential atrophy of type II
muscle fibers with 2 wk of detraining (34), which would reduce the
relative proportion of type II MHC. Thus the present findings may not
extend to highly trained athletes because of differences in initial
muscle fiber size and level of conditioning.”
However, beginners (recreational athletes) tend to lose muscle mass at a slower pace during detraining. The study group consisted of sedentary males and found that atrophy and decreased strength occurred aft 12 weeks.
“After 3 mo of detraining these gains were lost…”
It appears that several factors contribute to the rate of muscle loss including age and current fitness level. Each person’s rate of muscle loss will vary based on these factors.
Taking time off (detraining), however, should not be looked upon as a bad thing. It can be beneficial because it gives your muscles (and your mind) much needed rest. And, kept to a minimum, planned rest periods should not negatively impact your ability to gain muscle mass in the future. What’s important is that you do not let the rest period interfere with your training goals.