How quickly can muscle mass be lost if a person who stops exercising?

A few weeks: You start to lose fitness after a week then muscle strength/endurance if not exercising for longer. If you are completely immobilized - like in a cast - those muscles will start to atrophy ( get smaller) after a week or so.

Loss of Muscle Mass: Muscle mass is loss at varying rates when we do not exercise regularly, but the general rule is 1% loss per day and 1/4% gain per day. This is why it is much easier to lose then gain it back. I am personally in the painful long process of regaining. Dr rich blake.