The effect of a workout is to raise MPS rates in the trained muscle fibers for approximately 48 hours afterwards. Beyond that, the MPS rate dips below the level required to maintain the fiber, unless it is used during normal physical activity. However, the decrease in the MPS rate during a period of not training is not as large as the increase after the workout. Therefore, we don’t lose all of the gains achieved by a workout within 48 hours.

To be specific, we don’t lose “muscles” but we do lose protein content from the muscle fibers that are controlled by high-threshold motor units, since these are the ones that are rarely used in daily life.

Yes, we do lose muscle size very quickly as a result of detraining. A month is sufficient to lose a large proportion of our previously-achieved gains, as shown in this study.