Q: If you stop working out, how long does it take to start losing muscle mass?

A: It will greatly depend on your diet and whether you lose weight or not. If you eat enough, with good amount of protein and don't lose weight, then you will lose little muscle mass even over long period of time like a year. You will lose some strength, but both the strength and whatever little muscle mass you lost would come back within a month of restarting training.

If you stop training and lose weight, then you'll start losing muscle mass much quicker. So the key is to eat well and not lose weight if you want to hang on to your muscle mass.

In all cases, it will be easier to regain the lost muscle mass than it was to build up in the first place.