Having been a natural bodybuilder for over 20 years, going through various cycles of bulking up, and cutting, as well as coaching many trainees I can tell you there are few simple answers to questions about body composition changes (in spite of what many self-styled ‘gurus’ will tell you). Yours is no different.
As I’m sure you are not surprised to learn, the first thing I will say in answer to your question is ‘it depends on many factors’. Off the top of my head here are a few; nutrition, rest, hormone levels, age, genetic propensity to gain and retain muscle, stress, lifestyle, and how long you trained before you stopped etc etc.
Of course many of these are multi-factorial and not mutually exclusive. For example your genetic makup is likely to affect your hormone levels and your ability to rest/recuperate.
For these reasons studying muscle loss (and gain) is a bit of a pseudo-science. As a research scientist myself I have studied much of the literature regarding strength training etc, and find a disturbing amount of it to be poorly conducted due simply to the multi-factorial nature of gaining/retaining muscle. How does one really control for all these variables and conclude that the findings of the study are related to the ‘apparent’ variable under consideration?
I would say that on a personal level you are very unusual. I’ve never heard of anyone retaining muscle for such a long period post cessation of training, and it makes me suspicious that perhaps your history of muscle building is more complex than first meets the eye. Meaning, that perhaps even you are unaware of the factors that led you to believe you have ‘built’ up a good physique. No criticism of you personally, but experience tells me that balanced judgment and bodybuilding are not happy bedfellows!
To give a more general, and yet anecdotal answer to your question, my experience and that of most of the people I have trained with is that in general people will probably lose about 10% of any gained strength (and muscle) per month of inactivity. So by about a year after you stop training you will somewhere close to where you began.
Of course all this can be messed up by any of the variables I mentioned earlier. Specifically, if you stopped training and started taking the right combination of supplements, drugs and food, then you would lose your muscle much more slowly.
Did you gain muscle very easily in the first place? Meaning, that you put on muscle and increased strength faster than most other trainees. My guess would be you are one of the ‘genetic elite’ that so many of us inferior beings look up to. 95% of people struggle to attain and maintain muscle, with a small number putting on muscle relatively easily. For people like you the general rules do not apply. You lucky devil!
Get back in the gym and make the most of it. Chicks dig muscle.