Hard to say how long. The muscle system can revert back to near (though not matching...that will take more future effort on your part) the top level (size & strength) it held before interrupting training. And fairly quickly. Has been debated if the former development was helped with drugs, than it may take longer. That I do not know. Anyway the body will seek that same prime lever of development as a natural choice or selection. If your going to hit it hard, with serious workouts, than probably with-in a 2 to 3 month range. Using your being a natural as a rule of thumb. All this in regard to personal diet, training, recovery time and mind set.
If a person is blessed with great genetics, than pretty darn fast. Have seem some of these guy's make impressive body changes day by day. Gaining muscular weight and losing fat goes alone with the process with these types. Of course, if your older now, than snapping back into shape will take a little longer.
Calling the term "muscle memory' slang is way too incorrect to even discuss. Good Luck.