The Center for Healthy Aging and the Department of Biomedical Sciences at the University of Copenhagen found it only takes two weeks of immobility for people to lose about one third of their muscular strength, putting them on the same level as someone forty or fifty years older.
The study was done to monitor muscle loss in people who were injured or sick. They found during periods of inactivity, young people lose more muscle mass than older people. In addition, fit people lost more muscle mass than unfit people.
From the press release:
"The more muscle mass you have, the more you’ll lose. Which means that if you’re fit and become injured, you’ll most likely lose more muscle mass than someone who is unfit, over the same period of time. But even though older people lose less muscle mass and their level of fitness is reduced slightly less than in young people, the loss of muscle mass is presumably more critical for older people, because it is likely to have a greater impact on their general health and quality of life." - Martin Gram, researcher at the Center for Healthy Aging and the Department of Biomedical Sciences
It takes three times the length of the period you were inactive to regain your former muscle mass. Simple cardio like cycling or walking isn’t enough either. You’ll have to include weight training.
Although complete immobility is not likely for most people, it shows the importance of a daily fitness routine to maintaining your muscle mass.