Excellent question.

Like any other habit or performance practice repeated over time, neural connections become more efficient signal conductors, more “solid” with repetition. Such is the case with what is commonly called muscle memory. You buy this quality with your resources of effort,

consistency, practice and time spent in the gym.

When you stop spending these above mentioned resources, the quality of specific neural adaptation, of muscle memory begins to lessen and then to eventually disappear with time. It appears to follow a relatively steep exponential loss pattern , seemingly to flatten out and gradually to shrink to a minimum level plotted against time and associated vagaries of the ageing process.

Of course, many other things influence how fast this neural adaptation and its associated advantages disappear: genetics, training history, age, lifestyle choices like nutrition, drug use, sleeping patterns, etc.

Unfortunately, what appears to be the the rule is as follows: harder to get it>less difficult to maintain it>easy to lose it = in most circumstances holds true.