There is a difference between the stoppage of muscle growth and detraining.
Detraining would only start to about 2 weeks after you stop stimulating a muscle.
What happens is that for around 36h after you stimulated a muscle hard enough, the rate of protein synthesis increases significantly above baseline and more than protein degradation. In normal circumstances, protein synthesis and protein breakdown are fairly balanced (although things like chronic cortisol increase can lead to a protein breakdown higher than protein synthesis).
Once your body comes back to that balanced state, you are no longer building muscle. Or if you are, it is at a non-significant rate. But you do not start to lose the muscle you built until after a longer period of de-training.